mfd: tps6105x: Fix possible NULL pointer access

tps6105 driver provides two cells. One is for GPIO and another one is
for selected mode depending on platform data. When tps6105x is used in
GPIO-only mode, this driver calls mfd_add_devices() with mfd_cell
.name == NULL. This value causes an oops in platform_device_register()
later.

The following patch adds a mfd_cell for each possible mode thereby
excluding .name assignment in runtime.

Signed-off-by: Denis Grigoryev <grigoryev@fastwel.ru>
Signed-off-by: Lee Jones <lee.jones@linaro.org>