mips: mtmips: fix dram size detection in dram_init

CONFIG_SYS_SDRAM_BASE points to cached memory, i.e. KSEG0, which is not
suitable for detecting memory size.

Replace CONFIG_SYS_SDRAM_BASE with KSEG1, and make dram_init() always do
memory size detection in any stage.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>