MIPS: BCM63xx: Fix SDRAM size computation for BCM6345

Instead of hardcoding the amount of available RAM, read the number of
effective multiples of 8MB from SDRAM_MBASE_REG.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3008/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>