soc: renesas: r8a774c0-sysc: Fix power request conflicts

Describe the location and contents of the SYSCEXTMASK register on
RZ/G2E, to prevent conflicts between internal and external power
requests.

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

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Link: https://lore.kernel.org/r/20190920143523.23125-1-geert+renesas@glider.be