boards: nxp: frdm_mcxn947: Updated Init Clock for LPTMR

Updated the clocks that get initialized for the MCXN947
when using the LPTMR. The LPTMR allows the user to select
and Input clock, however said input clock must be
initialized before the user can select it.
Update description for clk-source in binding file.

Signed-off-by: Emilio Benavente <emilio.benavente@nxp.com>