Add aml8726-m3 usbclock support

[john-feith.com]

Summary

The usbclock generator on the earlier amlogic chip is a bit different
so add a new driver.

Test Plan

Tested on YYHD18 (aml8726-m3).

[ganbold]

sys/arm/amlogic/aml8726/aml8726_usb_phy-m3.c

254

Maybe 4 space tab?

398

Same here.

410

maybe only a tab inside DEVMETHOD instead of space and tab?

[ganbold]

sys/arm/amlogic/aml8726/aml8726_usb_phy-m3.c

213

Is it intentional here to set err and then later check in out of the loop?

[john-feith.com]

I've made the suggested changes (as well as adding break to the corresponding code in the m6 usb clock code).

[john-feith.com]

I've responded to the inline comments.

sys/arm/amlogic/aml8726/aml8726_usb_phy-m3.c

213

Yes ... memory is allocated prior to the loop by OF_getencprop_alloc which is freed after the loop
(in both success as well as error states). Technically we can break out of the loop when err is set
so I've added the missing break, though no harm came from contining the loop.