rockchip: sdram-common: fix wrong size for 4GB in 32bit SoC

rockchip: sdram-common: fix wrong size for 4GB in 32bit SoC

This is workaround for issue we can't get correct size for 4GB ram
in 32bit system and available before we really need ram space
out of 4GB, eg.enable ARM LAPE(rk3288 supports 8GB ram).
The size of 4GB is '0x1 00000000', and this value will be truncated
to 0 in 32bit system, and system can not get correct ram size.
Rockchip SoCs reserve a blob of space for peripheral near 4GB,
and we are now setting SDRAM_MAX_SIZE as max available space for
ram in 4GB, so we can use this directly to workaround the issue.

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