ARM: mvebu: x530: use MV_DDR_FREQ_SAR

MV_DDR_FREQ_SAR lets the DDR frequency be determined by hardware
strapping. This also has the side effect of running the DDR clock in
synchronous mode with the CPU core clock rather than from an independent
PLL. We've seen this improve reliability in operation across a number of
boards and temperature ranges.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>