Add driver for marvell, armada-3700 oscillator

Driver registers new clock device. Clock frequency is set depending
on tenth bit's value obtained from syscon register. Full information
about the clock is dumped if bootverbose is enabled.

Driver was tested on EspressoBin.

Reviewed by: manu
Obtained from: Semihalf
Differential revision: https://reviews.freebsd.org/D32292