common: board_r: include asm-generic/gpio.h

Not all architectures define <asm/gpio.h> and even on those that do, the
header cannot be included for all boards without causing various build
failures.

Since common/board_r.c only needs gpio_hog_probe_all() declaration, include
<asm-generic/gpio.h> and drop the associated ifdef.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>