rockchip: dts: rk3288: correct sdram setting for miniarm

miniarm board use lpddr3

Signed-off-by: Jacob Chen <jacob2.chen@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>
Added 'rockchip:' prefix to subject:
Signed-off-by: Simon Glass <sjg@chromium.org>

Change-Id: I84c3679dab2dbd8d01c1ebfd22220946d07c03cd