gpio-tegra: Do not create the debugfs entry by default

The tegra gpio driver creates the debugfs entry irrespective of
whether the device exists or not. This is enabled on an arm64_defconfig
and leaves an entry in debugfs on all platforms where it is not
useful. This patch fixes the issue by creating the entry only when
a device exists.

Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>