mfd: Build rtc5t583 only if I2C config is selected to y.

Fixing build error reported by Stephen Rothwell:

drivers/built-in.o: In function `rc5t583_i2c_init':
rc5t583.c:(.init.text+0xb3db): undefined reference to `i2c_register_driver'
drivers/built-in.o: In function `rc5t583_i2c_probe':
rc5t583.c:(.devinit.text+0x8fa0): undefined reference to `regmap_init_i2c'
drivers/built-in.o: In function `rc5t583_i2c_exit':
rc5t583.c:(.exit.text+0x708): undefined reference to `i2c_del_driver'

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>