ARM: rmobile: salvator-x: Add DVFS and PMIC support

Add support for rebooting the board using the ROHM BD9571MWV I2C PMIC,
but keep the CPU reboot option as a fallback.

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