soc: tegra: Remove redundant $(CONFIG_ARCH_TEGRA) in Makefile

Kbuild descends into drivers/soc/tegra/ only when CONFIG_ARCH_TEGRA
is enabled. (see drivers/soc/Makefile)

$(CONFIG_ARCH_TEGRA) in drivers/soc/tegra/Makefile always evaluates
to 'y'.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>