ARM: shmobile: r8a7740 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.

Add a minimal device node for the Coresight-ETM hardware block, and hook
it up to the D4 PM domain, so the R-Mobile System Controller driver can
keep the domain powered, until the new Coresight code handles runtime
PM.

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