rockchip: rk3288: sdram: style fixes from rk3188 sdram review

The sdram IP blocks used on rk3066, rk3188 and rk3288 are very similar
and we want to unify things once all 3 work as expected.
Therefore try to keep the rk3288 sdram driver in line by applying the
general review comments received for the rk3188 variant to it as well.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Simon Glass <sjg@chromium.org>