usb: typec: add typec_find_pwr_opmode

This patch adds a function that converts power operation mode string into
power operation mode value.

It is useful to configure power operation mode through device tree
property, as power capabilities may be linked to hardware design.

Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/20200924090049.9041-3-amelie.delaunay@st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>