dt-bindings: consolidate USB PHYs in bindings/phy

Move USB PHY bindings under usb directory to phy directory which already
contains other USB PHY bindings.

The Samsung USB PHY binding is obsolete and can be removed.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>