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

drivers/soc/renesas/r8a774c0-sysc.c

@@ -6,6 +6,7 @@
  * Based on Renesas R-Car E3 System Controller
  */
 
+#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 r8a774c0_sysc_info __initconst = {
 	.init = r8a774c0_sysc_init,
 	.areas = r8a774c0_areas,
 	.num_areas = ARRAY_SIZE(r8a774c0_areas),
+	.extmask_offs = 0x2f8,
+	.extmask_val = BIT(0),
 };