ARM: OMAP2+: Configure voltage controller for retention

Similar to existing omap3_vc_set_pmic_signaling(), let's add omap4
specific omap4_vc_set_pmic_signaling(). This allows the configured
devices to enable voltage controller for retention later on during
init.

Cc: Merlijn Wajer <merlijn@wizzup.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>