Documentation: dt: Add 'synopsys' to vendor prefixes list

The device tree binding for the Synopsys DWC3 USB controller
IP core currently uses the alternative 'synopsys' (as opposed
to 'snps') as the vendor prefix for its compatible string. Add
it to the list of prefixes in order to satisfy checkpatch.

Change-Id: Ie1799f93d36334197cc2f29986e77eb591f3553a
Signed-off-by: Jack Pham <jackp@codeaurora.org>