MIPS: BCM63xx: Define internal registers offsets of the SPI controller

MIPS: BCM63xx: Define internal registers offsets of the SPI controller

BCM6338, BCM6348, BCM6358 and BCM6368 basically use the same SPI controller
though the internal registers are shuffled, which still allows a common
driver to drive that IP block.

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/3318/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>