gpio: GPIO_SAMA5D2_PIOBU should depend on ARCH_AT91

The SAMA5D2 PIOBU is only present on some AT91/Microchip SoCs.  Hence
add a dependency on ARCH_AT91, to prevent asking the user about this
driver when configuring a kernel without AT91/Microchip SoC support.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>