mfd: tps65218: Use mfd_add_devices instead of of_platform_populate

mfd_add_devices enables parsing device tree nodes without compatibles
for regulators and gpio modules. Replace of_platform_populate with
mfd_add_devices. mfd_cell currently is populated with regulators,
gpio and powerbutton.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>