MIPS: BCM63XX: move UART register definitions

Move the BCM63XX UART driver definitions to
include/linux/serial_bcm63xx.h such that we do not rely on the MIPS
BCM63XX code to provide these for us.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6202/