OMAP2: PM: check UART status before trying to idle

As is done on OMAP3, check omap_uart_can_sleep() as one of the
pre-conditions for entering the idle loop.  Without this check,
entering idle introduces large latencies on active UARTs, and is
especially noticable on serial console.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>