armv8: ls1012a: Update Refresh cycle for DDR

Refresh cycle value must be selected based on the frequency
of DDR. tREFI = 7.8 us as per JEDEC. The value for MDREF[REF_CNT]
should be based on round up (tREFI/tCK) formula. For 500MHz, mdref
value should be 0x0f3c8000.
Signed-off-by: NCalvin Johnson <calvin.johnson@nxp.com>
Signed-off-by: NPrabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reviewed-by: NYork Sun <york.sun@nxp.com>

include/fsl_mmdc.h

@@ -43,7 +43,7 @@
 
 #define CONFIG_SYS_MMDC_CORE_PWR_SAV_CTRL_STAT	0x00001067
 
-#define CONFIG_SYS_MMDC_CORE_REFRESH_CTL	0x103e8000
+#define CONFIG_SYS_MMDC_CORE_REFRESH_CTL	0x0f3c8000
 
 #define START_REFRESH				0x00000001