arm: kirkwood: Change naming of dram functions from km_foo() to mvebu_foo()

arm: kirkwood: Change naming of dram functions from km_foo() to mvebu_foo()

Additionally the SDRAM address decoding register address is not hard coded
in the C code any more. A define is introduced for this base address.

This makes is possible to use those gpio functions from other MVEBU SoC's
as well.

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Luka Perkov <luka@openwrt.org>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>