docs: migration-guide-3.5: LPC55XXX Clock Init

The inclusion of this note in the migration guide
explains the clocking change that occured in the
LPC55XXX soc as well as the added Kconfig that toggles
the PLL1 from being initialized. Also updated the
lpc board docs to state the correct System
Clock Value.

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