spi: ath79: Enable support for compile test

To allow building this driver in compile test we need to remove all
dependency on headers from arch/mips/include. To allow this we
explicitly define all the registers locally instead of using
ar71xx_regs.h and we move the platform data struct definition to
include/linux/platform_data/spi-ath79.h.

Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>