pinctrl: tegra-xusb: Fix allocation of pins

Commit e5b3b2d9 ("pinctrl: allows not to define the get_group_pins
operation") allows pin controllers not to register the get_group_pins()
function. However, a side-effect of not registering this function is
that pins are not allocated and potentially multiple devices could
attempt to configure the same pins [1]. Although this problem exists in
the pinctrl core, because only a few devices are impacted by this, fix
this for tegra-xusb by adding the get_group_pins() function.

Please note that in addition to adding the get_group_pins() functions
the pins/lanes for the tegra-xusb also need to be registered when
calling pinctrl_register(). This also allows the current pinmux state
to be viewed by the debugfs node "pinmux-pins" for the tegra-xusb pad
controller.

[1] http://www.spinics.net/lists/linux-gpio/msg05810.html



Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>