ARM: OMAP: dmtimers: Fix locking issue in omap_dm_timer_request*()

Calling omap_dm_timer_prepare while the spinlock is held is not
allowed as sleeping functions are called later on during the
preparation (namely within clk_get()).

dm_timer_lock is only required for protecting the
omap_timer_list. After the timer is marked as reserved, the lock is no
longer needed and should be freed.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>
Cc: Tarun Kanti DebBarma <tarun.kanti@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>