rockchip: arm64: rk3399: revise timeout-handling for DRAM PHY lock

Revise the loop watching for a timeout on obtaining a DRAM PHY lock to
clearly state a timeout in milliseconds and use get_timer (based on
the ARMv8 architected timer) to detect a timeout.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>