mfd: ssbi: Use devm_of_platform_populate()

Use devm_of_platform_populate() instead of of_platform_populate()
to be sure that of_platform_depopulate() is called when removing
the driver.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>