arm: mxs: Wait when disabling VDDMEM current limiter

According to i.MX23 datasheet Table 32-17, we must wait for the supply
to settle before disabling the current limiter. Indeed, not waiting a
little here causes the system to crash at times.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>