mx51evk: Add RTC support

MX51EVK has a MC13892 PMIC. Add RTC support.

Tested via 'date' command that reads the PMIC RTC registers:

MX51EVK U-Boot > date
Date: 1970-01-01 (Thursday)    Time:  0:40:35

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>

include/configs/mx51evk.h

@@ -79,6 +79,7 @@
 #define CONFIG_FSL_PMIC_CLK	2500000
 #define CONFIG_FSL_PMIC_MODE	(SPI_MODE_0 | SPI_CS_HIGH)
 #define CONFIG_FSL_PMIC_BITLEN	32
+#define CONFIG_RTC_MC13XXX
 
 /*
  * MMC Configs
@@ -124,6 +125,8 @@
 
 #undef CONFIG_CMD_IMLS
 
+#define CONFIG_CMD_DATE
+
 #define CONFIG_BOOTDELAY	3
 
 #define CONFIG_ETHPRIME		"FEC0"