gpio: pl061 depends on ARM

Commit dece904d (gpio: pl061: use chained_irq_* functions in irq
handler) introduced chained_irq_enter/exit, which is only
available for arch/arm and the driver won't compile elsewhere.

The dependency is thus made explicit, because AMBA is used in the x86
world by a PCI-to-AMBA bridge, to be submitted.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Giancarlo Asnaghi <giancarlo.asnaghi@st.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>