MIPS: BCM63XX: use linux/serial_bcm63xx.h

Update the early_printk code to include linux/serial_bcm63xx.h which
provides the definitions for the UART block registers. While at it,
remove the inclusion of serial_bcm63xx.h which was just there to allow
smooth transition.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6203/