arm: mvebu: Print CPU and SDRAM frequency upon startup

With this patch, the CPU and the DDR frequencies will get printed in the
U-Boot startup messages. Resulting in such a log:

U-Boot 2016.01-rc2-00188-gb8eeaec-dirty (Dec 21 2015 - 12:32:35 +0100)

SoC:   MV78460-B0 at 1600 MHz
I2C:   ready
DRAM:  4 GiB (800 MHz, ECC not enabled)
...

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>