mach-omap2/powerdomain.c: convert rwsem to rwlock

The generic rwsem implementation of down_read() and down_write() does not
save and restore interrupt state.  This causes powerdomain code to
inadvertently enable interrupts early in the boot process, causing
init/main.c to complain.   This patch converts powerdomain locking to
r-w spinlocks instead.

I'm also curious to know if this fixes the BeagleBoard boot problem.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>