docs: phy: convert samsung-usb2.txt to ReST format

docs: phy: convert samsung-usb2.txt to ReST format

In order to merge it into a Sphinx book, we need first to
convert to ReST.

While this is not part of any book, mark it as :orphan:, in order
to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>