MIPS: BCM63XX: remove duplicate spi register definitions

BCM6338 and BCM6348, and BCM6358 and everything after that share the
same register layout. To not have to redefine them for each new chip
and keep the code size small, only use the definitions for the first
chip with the certain layout.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5006/
Acked-by: John Crispin <blogic@openwrt.org>