arm64: mvebu: a8k: Add support for NAND clock get

Implement mvebu_get_nand_clock call for A8K family.
This function is used by PXA3XX NAND driver.

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Igal Liberman <igall@marvell.com>
Cc: Nadav Haklai <nadavh@marvell.com>
Reviewed-by: Stefan Roese <sr@denx.de>