usb: host: ehci-generic: Make resets and clocks optional

The generic EHCI binding does not *require* resets and clocks
properties, and indeed for instance the Allwinner A20 SoCs does not
need or define any resets in its DT.

Don't easily give up if clk_get_bulk() or reset_get_bulk() return an
error, but check if that is due to the DT simply having no entries for
either of them.

This fixes USB operation on all boards with an Allwinner A10 or A20 SoC,
which were reporting an error after commit ba96176a:
=======================
Bus usb@1c14000: ehci_generic usb@1c14000: Failed to get resets (err=-2)
probe failed, error -2
=======================

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>