soc: renesas: r8a77990-sysc: Fix power request conflicts

Describe the location and contents of the SYSCEXTMASK register on R-Car
E3, 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: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Link: https://lore.kernel.org/r/20190828113618.6672-8-geert+renesas@glider.be