ACPI: Change acpi_ioremap_lock into a mutex

There's no reason why acpi_ioremap_lock has to be a spinlock,
because all of the functions it is used in may sleep anyway and
there's no reason why it should be locked with interrupts off.
Use a mutex instead (that's going to allow us to put some more
operations under the lock later).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>