OMAP3: PM: handle PER/NEON/CORE in idle

Expand the powerdomains handled in the idle path to include PER, NEON
and CORE.  This includes properly clearing the previous powerstates,
linking NEON state to MPU state and calling the UART prepare functions
for only the appropraite powerdomain transitions (CORE for UART1,2,
PER for UART3.)

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>