gpio: dwapb: fix compile errors

Whereas the DWAPB driver does not really depend on the ARM
architecture, it uses [readl|writel]_relaxed() not found on
arch such as Blackfin, so restrict this to ARM until there is
another architecture that can make use of it.

It is also using the of_node of the gpiochip, so fix this
too by requiring OF_GPIO.

All error/warnings:

make.cross ARCH=blackfin
drivers/gpio/gpio-dwapb.c: In function 'dwapb_irq_handler':
drivers/gpio/gpio-dwapb.c:91:2: error: implicit declaration of function 'readl_relaxed' [-Werror=implicit-function-declaration]
drivers/gpio/gpio-dwapb.c: In function 'dwapb_configure_irqs':
drivers/gpio/gpio-dwapb.c:212:32: error: 'struct gpio_chip' has no member named 'of_node'
drivers/gpio/gpio-dwapb.c:221:16: error: 'struct gpio_chip' has no member named 'of_node'
drivers/gpio/gpio-dwapb.c: In function 'dwapb_gpio_add_port':
drivers/gpio/gpio-dwapb.c:331:14: error: 'struct gpio_chip' has no member named 'of_node'
cc1: some warnings being treated as errors

Cc: Jamie Iles <jamie@jamieiles.com>
Cc: Alan Tull <atull@altera.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>