drivers: mipi_dsi: dsi_mcux_2l add msg flag for low power mode

Previous version of dsi_mcux_2l hardcoded some MIPI DSI
transfers to use high speed mode but others used low power mode.

Now dsi_mcux_2l will use high speed mode by default for all
transfers unless a new msg flag is set to indicate the
transfer must use low power mode. Note that the new flag
is different than the existing MIPI_DSI_MODE_LPM flag, which
so far only applied to cmd messages sent in video mode,
or could be interpreted as for all messages, but would not
allow per message mode control.

This new message flag allows client to control transfer
mode per message transfer.

Signed-off-by: Mike J. Chen <mjchen@google.com>