pinctrl: tegra: Fix build dependency

I got below build error:
ERROR: "tegra_xusb_padctl_legacy_probe"
 [drivers/phy/tegra/phy-tegra-xusb.ko] undefined!
with below build configuration:
CONFIG_ARCH_TEGRA=y
CONFIG_PINCTRL_TEGRA_XUSB=y
CONFIG_PHY_TEGRA_XUSB=y

The problem is below line in drivers/pinctrl/Makefile
obj-$(CONFIG_PINCTRL_TEGRA)     += tegra/

So even CONFIG_PINCTRL_TEGRA_XUSB=y is set, kbuild still does not compile
the code in drivers/pinctrl/tegra folder if !CONFIG_PINCTRL_TEGRA.

phy-tegra-xusb.c does not use any symbol from pinctrl-tegra.c,
so build pinctrl-tegra.c only when CONFIG_PINCTRL_TEGRA is set.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>