MIPS: ath79: add common SPI controller device

Several boards are using the built-in SPI controller of the
AR71XX/AR724X/AR913X SoCs. This patch adds common platform_device
and helper code to register it. Additionally, the patch registers
the SPI bus on the PB44 board.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Luis R. Rodriguez <lrodriguez@atheros.com>
Cc: Cliff Holden <Cliff.Holden@Atheros.com>
Cc: Kathy Giori <Kathy.Giori@Atheros.com>
Patchwork: https://patchwork.linux-mips.org/patch/1956/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>