x86: rtc: mc146818: Add helpers to read/write CMOS RAM

On x86 we use CMOS RAM to read and write some settings. Add basic support
for this, including access to registers 128-255.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/rtc.h

@@ -50,6 +50,38 @@ void to_tm (int, struct rtc_time *);
 unsigned long mktime (unsigned int, unsigned int, unsigned int,
 		      unsigned int, unsigned int, unsigned int);
 
+/**
+ * rtc_read8() - Read an 8-bit register
+ *
+ * @reg:	Register to read
+ * @return value read
+ */
+int rtc_read8(int reg);
+
+/**
+ * rtc_write8() - Write an 8-bit register
+ *
+ * @reg:	Register to write
+ * @value:	Value to write
+ */
+void rtc_write8(int reg, uchar val);
+
+/**
+ * rtc_read32() - Read a 32-bit value from the RTC
+ *
+ * @reg:	Offset to start reading from
+ * @return value read
+ */
+u32 rtc_read32(int reg);
+
+/**
+ * rtc_write32() - Write a 32-bit value to the RTC
+ *
+ * @reg:	Register to start writing to
+ * @value:	Value to write
+ */
+void rtc_write32(int reg, u32 value);
+
 /**
  * rtc_init() - Set up the real time clock ready for use
  */