staging: omap-thermal: switch mutex to spinlock inside omap-bandgap

Because there is a need to lock inside IRQ handler, this patch
changes the locking mechanism inside the omap-bandgap.[c,h] to
spinlocks. Now this lock is used to protect omap_bandgap struct
during APIs exposed (possibly used in sysfs handling functions)
and inside the ALERT IRQ handler.

Because there are registers shared among the sensors, this lock
is global, not per sensor.

Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>