ssb: add ssb_chipco_gpio_pull{up,down}

Add functions to access the GPIO registers for pullup and pulldown.
These are needed for handling gpio registration.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: http://patchwork.linux-mips.org/patch/4589


Acked-by: Florian Fainelli <florian@openwrt.org>