msm: qpnp-power-on: protect spon_list with spinlock instead of mutex

As part of the msm panic path, qpnp_pon_system_pwr_off() is called
from IRQ disabled context and currently results in a BUG() since
the function acquires the mutex protecting spon_list. Switch
to using a spinlock as synchronization mechanism to protect
spon_list since it is safe for use in atomic context.

Change-Id: I0e3fbcc7e2419db8f779b0ff30cd6b5f41bf73bc
Signed-off-by: Osvaldo Banuelos <osvaldob@codeaurora.org>