i2c: jz4780: Drop dependency on MACH_JZ4780

Depending on MACH_JZ4780 prevent us from creating a generic kernel that
works on more than one MIPS board. Instead, we just depend on MIPS being
set.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>