rcar/module: add software reset as a system reset

This patch add software reset for the system that does not use PMIC as
system reset source. System reset source can be selected on compilation by
macro definition (RCAR_SOFTWARE_RESET). The default is PMIC.

Unnecessary conditional compilation has been deleted since it targets
system boards equipped with PMIC.

Signed-off-by: Tsutomu Muroya <tsutomu.muroya.jy@bp.renesas.com>
Signed-off-by: Nicolas Royer <nroyer@baylibre.com>