soc: renesas: rcar-sysc: Prepare for fixing power request conflicts

Recent R-Car Gen3 SoCs added an External Request Mask Register to the
System Controller (SYSC).  This register allows to mask external power
requests for CPU or 3DG domains, to prevent conflicts between powering
off CPU cores or the 3D Graphics Engine, and changing the state of
another power domain through SYSC, which could lead to CPG state machine
lock-ups.

Add support for making use of this register.  Take into account that the
register is optional, and that its location and contents are
SoC-specific.

Note that the issue fixed by this cannot happen in the upstream kernel,
as upstream has no support for graphics acceleration yet.  SoCs lacking
the External Request Mask Register may need a different mitigation in
the future.

Inspired by a patch in the BSP by Dien Pham <dien.pham.ry@renesas.com>.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Link: https://lore.kernel.org/r/20190828113618.6672-2-geert+renesas@glider.be