pinctrl: sirf: add pin group for USP0 with only RX or TX frame sync

USP0 has multiple functions, and has RX and TX frame sync signals,
for some scenarios like audio PCM, we don't need both of them.
so here we add two possibilities for USP0 only holding one of TX
and RX frame sync.

Signed-off-by: Rongjun Ying <Rongjun.Ying@csr.com>
Signed-off-by: Barry Song <Barry.Song@csr.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>