usb: dwc3: add vendor prefix against used dwc3 flags

Currently there is no prefix used with dwc3 flags like
tx-fifo-size, host-only-mode, nominal-elastic-buffer and
core_reset_after_phy_init (rename as core-reset-after-phy-init).
This change adds snps as prefix to reflect that those are used
usb core vendor's feature. This change also makes required
changes with all devicetree files where it is applicable.

Change-Id: I9fa5c697491a3cde4a53df4fe6d7d2a8448353a5
Signed-off-by: Mayank Rana <mrana@codeaurora.org>