arm: mxs: Wait for DRAM to start

Instead of waiting for a fixed period of time and hoping for the best
that the DRAM will start, read back an EMI status register which tells
us exactly when the DRAM started.

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