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>