feat(nxp-clk): implement set_rate for oscillators

The set_rate callback will now be applied to FIRC, FXOSC, and SIRC
oscillators. It is a prerequisite for the upcoming commits that will
utilize this capability.

Change-Id: I82d1545c63b3e15497c1c002ff9ec0d7bf990aa0
Signed-off-by: Ciprian Costea <ciprianmarian.costea@nxp.com>
Signed-off-by: Ghennadi Procopciuc <ghennadi.procopciuc@nxp.com>