pinctrl: sirf: add USB1/UART1 pinmux usb/uart share

dn and dp of USB1 can share with UART1(UART1 can route rx,tx to dn and dp pins of USB1).
here we add this pinmux capability.
USB1/UART1 mode selection has dedicated control register in RSC module, here we attach
the register offset of private data of related pin groups.

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