dts: nxp_lpc_lpadc: Make clk props optional

These properties should eventually be removed from this binding as they
have been introduced to control soc specific clock trees and don't
correlate to anything in the IP, but for now just make them not required
and remove them from DT for SOCs that don't even use them.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>