sx150x-pinctrl: Avoid i2c transfer during kexec

machine_crash_shutdown() calls irq_bus_sync_unlock() with local irq
disabled. sx150x_irq_bus_sync tries to transfer over i2c bus and hangs
because of disabled irqs.

Add panic handler, so that sx150x_irq_bus_sync_unlock can
return before write to iomap register. Mulitple instances of sx150x
device may be present, hence a global flag to indicate
machine_crash_shutdown() context.

Change-Id: If992f730ba36b81b2a231f3605673e8bae278d16
Signed-off-by: Akshay Adiga <aadiga@codeaurora.org>