mfd: twl6030: add software power off control

System can be shutdown from userspace by populating
pm_power_off hook for the platform

The hook is twl6030(pmic) specific here

Change-Id: I8821bb734e3171428d9380d352332608ae074f48
Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>

Conflicts:
	drivers/mfd/Kconfig
	drivers/mfd/Makefile