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

drivers/soc/renesas/r8a77990-sysc.c

@@ -5,6 +5,7 @@
  * Copyright (C) 2018 Renesas Electronics Corp.
  */
 
+#include <linux/bits.h>
 #include <linux/bug.h>
 #include <linux/kernel.h>
 #include <linux/sys_soc.h>
@@ -50,4 +51,6 @@ const struct rcar_sysc_info r8a77990_sysc_info __initconst = {
 	.init = r8a77990_sysc_init,
 	.areas = r8a77990_areas,
 	.num_areas = ARRAY_SIZE(r8a77990_areas),
+	.extmask_offs = 0x2f8,
+	.extmask_val = BIT(0),
 };