doc: arch: arm64: describe pagetable debugging

Add some brief documentation on using dump_pagetables() to print out
U-Boot's pagetables during boot.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>

doc/arch/arm64.rst

@@ -48,6 +48,55 @@ Notes
 6. CONFIG_ARM64 instead of CONFIG_ARMV8 is used to distinguish aarch64 and
    aarch32 specific codes.
 
+MMU
+---
+
+U-Boot uses a simple page table for MMU setup. It uses the smallest number of bits
+possible for the virtual address based on the maximum memory address (see the logic
+in ``get_tcr()``). If this is less than 39 bits, the MMU will use only 3 levels for
+address translation.
+
+As with all platforms, U-Boot on ARM64 uses a 1:1 mapping of virtual to physical addresses.
+In general, the memory map is expected to remain static once the MMU is enabled.
+
+Software pagetable walker
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+It is possible to debug the pagetable generated by U-Boot with the built in
+``dump_pagetable()`` and ``walk_pagetable()`` functions (the former being a simple
+wrapper for the latter). For example the following can be added to ``setup_all_pgtables()``
+after the first call to ``setup_pgtables()``:
+
+.. code-block:: c
+
+    dump_pagetable(gd->arch.tlb_addr, get_tcr(NULL, NULL));
+
+.. kernel-doc:: arch/arm/cpu/armv8/cache_v8.c
+   :identifiers: __pagetable_walk pagetable_print_entry
+
+The pagetable walker can be used as follows:
+
+.. kernel-doc:: arch/arm/include/asm/armv8/mmu.h
+   :identifiers: pte_walker_cb_t walk_pagetable dump_pagetable
+
+This will result in a print like the following:
+
+.. code-block:: text
+
+    Walking pagetable at 000000017df90000, va_bits: 36. Using 3 levels
+    [0x17df91000]                   |  Table |               |
+      [0x17df92000]                 |  Table |               |
+        [0x000001000 - 0x000200000] |  Pages | Device-nGnRnE | Non-shareable
+      [0x000200000 - 0x040000000]   |  Block | Device-nGnRnE | Non-shareable
+    [0x040000000 - 0x080000000]     |  Block | Device-nGnRnE | Non-shareable
+    [0x080000000 - 0x140000000]     |  Block | Normal        | Inner-shareable
+    [0x17df93000]                   |  Table |               |
+      [0x140000000 - 0x17de00000]   |  Block | Normal        | Inner-shareable
+      [0x17df94000]                 |  Table |               |
+        [0x17de00000 - 0x17dfa0000] |  Pages | Normal        | Inner-shareable
+
+For more information, please refer to the additional function documentation in
+``arch/arm/include/asm/armv8/mmu.h``.
 
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