ARM: da850-evm: add a fixed regulator for the UI board IO expander

Without this regulator the tca6416 GPIO expander on the UI board can't
be probed in board file mode and we're not getting VPIF IRQs.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>