[POWERPC] drivers/of/of_i2c.c: Add MODULE_LICENSE

After commit 585468e5


([POWERPC] i2c: Fix build breakage introduced by OF helpers)
drivers/of/of_i2c.c needs a MODULE_LICENSE.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>