ARM: shmobile: sh73a0 dtsi: Add PM domain support

Add a device node for the System Controller, with subnodes that
represent the hardware power area hierarchy.
Hook up all devices to their respective PM domains.

Note that unlike on R-Mobile A1 (r8a7740), PM domain D4 can be powered
down without ill effects on s2ram behavior, just like on SH-Mobile AP4
(sh7372).  Hence we can postpone adding a (minimal) device node for the
Coresight-ETM hardware block.

The System Controller is also used by the R-Mobile Reset driver, which
can now restart the system.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>