pinctrl: sirf: add lost USP-based UART pin groups for prima2

USP(Universal Serial Ports) can be UART as commit 5df83111,
this patch defines the USP-based UART function pin groups for prima2.

Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>