Improve HW type checking in mv_ehci driver

This patch adds hwtype parameter which keeps information about hardware
revision of Marvell EHCI controller. It allows to replace multiple
calls to ofw_bus_is_compatible with comparing hwtype value during driver
initialization.

Submitted by: Patryk Duda <pdk@semihalf.com>
Suggested by: ian
Obtained from: Semihalf
Sponsored by: Semihalf