ARM: rmobile: Rename r8a774a1_beacon_defconfig to rzg2_beacon_defconfig

Now that the three M/N/H variants can be built from one config,
rename the defconfig file to be more generic since it supports multiple
RZ/G2.

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/board/beacon/beacon-rzg2m/MAINTAINERS b/board/beacon/beacon-rzg2m/MAINTAINERS
index 45c269d..77c4057 100644
--- a/board/beacon/beacon-rzg2m/MAINTAINERS
+++ b/board/beacon/beacon-rzg2m/MAINTAINERS
@@ -3,6 +3,4 @@
 S:	Maintained
 F:	board/beacon/beacon-rzg2m/
 F:	include/configs/beacon-rzg2m.h
-F:	configs/r8a774a1_beacon_defconfig
-F:	configs/r8a774b1_beacon_defconfig
-F:	configs/r8a774e1_beacon_defconfig
+F:	configs/rzg2_beacon_defconfig