Add driver for "generic-ohci" as defined by FDT.

Add driver for "generic-ohci" as defined by FDT.
If platform support EXT_RESOURCES, clocks and resets are handled out of
the box.
If not driver can be subclassed using the generic_usb interface.
generic_usb name was choosed because at one point I'll add generic-ehci
FDT driver.

Reviewed by:	jmcneill, hselasky
Approved by:	andrew (mentor)
Differential Revision:	https://reviews.freebsd.org/D5481