gpio: sifive: select IRQ_DOMAIN_HIERARCHY rather than depend on it

This is the only driver in the kernel source tree that depends on
IRQ_DOMAIN_HIERARCHY instead of selecting it. Since it is not a
visible Kconfig symbol, depending on it (expecting a user to
set/enable it) doesn't make much sense, so change it to select
instead of "depends on".

Fixes: 96868dce ("gpio/sifive: Add GPIO driver for SiFive SoCs")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: linux-gpio@vger.kernel.org
Cc: Thierry Reding <treding@nvidia.com>
Cc: Greentime Hu <greentime.hu@sifive.com>
Cc: Yash Shah <yash.shah@sifive.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>