i2c: tegra: allow DVC support to be compiled out

Save a bit of code for newer Tegra platforms by compiling out
DVC's I2C mode support that's used only for Tegra2.

$ size i2c-tegra.o
    text    data     bss     dec     hex filename
-  11381     292       8   11681    2da1 i2c-tegra.o
+  10193     292       8   10493    28fd i2c-tegra.o

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>