MIPS: BCM63xx: Define SPI register sizes.

There are two distinct sizes for the SPI register depending on the SoC
generation (6338 & 6348 vs 6358 & 6368).

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: grant.likely@secretlab.ca
Cc: spi-devel-general@lists.sourceforge.net
Patchwork: https://patchwork.linux-mips.org/patch/3314/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>