tty: serial: bcm63xx_uart: use linux/serial_bcm63xx.h

Now that the UART block defines have been moved to a separate file,
include that one and do not longer rely on the MIPS-specific
bcm63xx_regs.h header file.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6204/