OMAP4 powerdomain: Support LOWPOWERSTATECHANGE for powerdomains

Some powerdomains in OMAP4 support a direct transition from one sleep
state to another deeper sleep state without having to wakeup the
powerdomain. This patch adds an api in the powerdomain framework to
set the LOWPOWERSTATECHANGE bit in PWRSTCTRL register.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>