fix(plat/marvell/a3720/uart): do external reset during initialization

Sometimes when changing UART clock from TBG to XTAL, UART HW enters into
some broken state. It does not transit characters from TX FIFO anymore
and TX FIFO stays always empty. TX FIFO reset does not recover UART HW
from this broken state.

Experiments show that external reset can fix UART HW from this broken
state.

TF-A fatal error handler calls console_a3700_core_init() function to
initialize UART HW. This handler may be called anytime during CPU
runtime, also when kernel is running.

U-Boot or Linux kernel may change UART clock to TBG to achieve higher
baudrates. During initialization, console_a3700_core_init() resets UART
configuration to default settings, which means that it also changes
UART clock from TBG to XTAL.

Do an external reset of UART via North Bridge Peripheral reset register
to prevent this UART hangup.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I8990bce24d1a6fd8ccc47a2cd0a5ff932fcfcf14

drivers/marvell/uart/a3700_console.S

@@ -69,6 +69,14 @@ func console_a3700_core_init
 	b.ne	2b
 
 4:
+	/* Reset UART via North Bridge Peripheral */
+	mov_imm	x4, MVEBU_NB_RESET_REG
+	ldr	w3, [x4]
+	bic	w3, w3, #MVEBU_NB_RESET_UART_N
+	str	w3, [x4]
+	orr	w3, w3, #MVEBU_NB_RESET_UART_N
+	str	w3, [x4]
+
 	/* Reset FIFO */
 	mov	w3, #UART_CTRL_RXFIFO_RESET
 	orr	w3, w3, #UART_CTRL_TXFIFO_RESET

include/drivers/marvell/uart/a3700_console.h

@@ -9,6 +9,7 @@
 #define A3700_CONSOLE_H
 
 #include <drivers/console.h>
+#include <platform_def.h>
 
 /* MVEBU UART Registers */
 #define UART_RX_REG		0x00