MIPS: BCM47xx: fix build with GENERIC_GPIO configuration

Since eb9ae7f2 (gpio: fix build error in include/asm-generic/gpio.h)
the generic version of gpio.h calls __gpio_{set,get}_value which we
do not define. Get rid of asm-generic/gpio.h and define the missing
stubs directly for BCM47xx to build.

Reported-by: Ralf Baechle <ralf@linux-mips.org>
To: linux-mips@linux-mips.org
CC: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Patchwork: https://patchwork.linux-mips.org/patch/2885/


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