MIPS: BCM63xx: Remove SPI2 register

This register was introduced with the support of the BCM6368 CPU in the idea
that its internal layout was different from the other CPUs SPI controller.
The controller is actually the same as the one present on BCM6358 so we can
remove this register and use the usual SPI register instead.

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/3316/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>