MIPS: BCM63xx: Remove BCM6345 hacks to read base boot address

Though BCM6345 does not technically have the same MPI register layout
than the other SoCs, reading the chip-select registers is done the same
way, and particularly for chip-select 0, which is the boot flash.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3009/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>