uboot-imx

am33xx: Add a function to query MPU voltage in uV

For the DM TPS65910 driver I'm working on, querying the MPU voltage
should return a value in uV. This value can then be used by the
regulator's standard function set_value to set the MPU voltage.

Signed-off-by: Felix Brack <fb@ltec.ch>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>