Add support to enable/disable both the EHCI and OHCI Allwinner clocks. This
adds a lock to ensure only a single device is accessing the hardware. A
reference count is added to only enable when we start to use the clock,
and to disable after we have finished needing the clock.

This was extracted from a larger review to add OHCI support to the
Allwinner SoCs.

Submitted by: Emmanuel Vadot <manu@bidouilliste.com>
Reviewed by: jmcneill
X-Differential Revision: https://reviews.freebsd.org/D5481