usb: dwc3: core: switch to snps,dwc3

all other drivers using Synopsys IPs with DT
have a compatible of snps,$driver, in order
to add consistency, we are switching over to
snps,dwc3 but keeping synopsys,dwc3 in the core
driver to maintain backwards compatibility.

New DTS bindings should NOT use synopsys,dwc3.

Signed-off-by: Felipe Balbi <balbi@ti.com>