drm: renesas: shmobile: Add support for Runtime PM

drm: renesas: shmobile: Add support for Runtime PM

The SH-Mobile LCD Controller is part of a PM Domain on all relevant SoCs
(clock domain on all, power domain on some).  Hence it may not be
sufficient to manage the LCDC module clock explicitly (e.g. if the
selected clock source differs from SHMOB_DRM_CLK_BUS).

Fix this by using Runtime PM for all clock handling.  Add an explicit
dependency on CONFIG_PM, which should already be met on all affected
platforms.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/e7359a64963bd9a4f1531c2beae850774ce140bc.1694767209.git.geert+renesas@glider.be