arm: mvebu: ac5: Define mvebu_get_nand_clock()

The NF_CLK for the AC5 SoC runs at 400MHz. There's no strapping
or gating require so just add a mvebu_get_nand_clock() that
returns this value.

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