ARM: rmobile: salvator-x: Rename the defconfig to match the SoC

Rename the salvator-x_defconfig to r8a7795_salvator-x_defconfig in
preparation for the r8a7796 support on salvator-x board.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>

board/renesas/salvator-x/MAINTAINERS

@@ -3,4 +3,4 @@ M:	Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
 S:	Maintained
 F:	board/renesas/salvator-x/
 F:	include/configs/salvator-x.h
-F:	configs/salvator-x_defconfig
+F:	configs/r8a7795_salvator-x_defconfig