rockchip: rk3188: Decode the actual amount of ram

There was still a static ram value set in the rk3188-board from the
time where we didn't have actual sdram init code.
Now the sdram init leaves the ram information in SYS_REG2 and we can
decode it similarly to the rk3288.

Right now we have two duplicates of that code, which is still ok and
doesn't really count as common code yet, but if we get a third copy
at some point from a newer soc, we should think about moving that to
a more general position.

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