regulator: pbias: Add PBIAS regulator for proper voltage switching on MMC1

In the TI SOCs a PBIAS cell exists to provide a bias voltage to the MMC1
IO cells. Without this bias voltage these I/O cells can not function
properly. The PBIAS cell is controlled by software.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>