irqchip: renesas-irqc: Add minimal runtime PM support

This is just enough to let pm_clk_*() enable the functional clock, and
manage it for suspend/resume, if present.
Before, it was assumed enabled by the bootloader or reset state.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Link: https://lkml.kernel.org/r/1426704961-27322-3-git-send-email-geert+renesas@glider.be
Signed-off-by: Jason Cooper <jason@lakedaemon.net>