pinctrl: sirf: add pin group for USP0 for atlas6

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.

commit 8385af02 only added this group for prima2, and missed
atlas6. This patch fixes it.

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