ARM: shmobile: r8a7740: Add A4SU pm domain support

Add support for the A4SU power domain.
This domain contains the USBPHY hardware block, which is currently not
used by any driver.

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