ddr: marvell: a38x: allow board specific clock out setup

DDR clock out might be unrelated to the number of active chip-select.
For example, the board might have two DDR components, but only one
chip-select. The clk_enable mask allows the board to enable DDR clocks
regardless of active chip-selects.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>