regulator: tps65218: force set power-up/down strobe to 3 for dcdc3

The reset value for this register seems broken on certain versions of
tps65218 chip, so make sure the dcdc3 settings is proper. Needed for
proper functionality of rtc+ddr / rtc-only modes.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>