pinctrl: vt8500: Change devicetree data parsing

Due to an assumption in the VT8500 pinctrl driver, the value passed
from devicetree for 'wm,pull' was not explicitly translated before
being passed to pinconf.

Since v3.10, changes to 'enum pin_config_param', PIN_CONFIG_BIAS_PULL_(UP/DOWN)
no longer map 1-to-1 with the expected values in devicetree.

This patch adds a small translation between the devicetree values (0..2)
and the enum pin_config_param equivalent values.

Cc: <stable@vger.kernel.org> # v3.10+
Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>