mfd: Fix STMPE I2c build failure

STMPE i2c is a bool and should depend on I2c=y.
That fixes:

drivers/built-in.o: In function `i2c_block_write':
stmpe-i2c.c:(.text+0xf4553): undefined reference to
+`i2c_smbus_write_i2c_block_data'
drivers/built-in.o: In function `i2c_block_read':
stmpe-i2c.c:(.text+0xf457f): undefined reference to
+`i2c_smbus_read_i2c_block_data'
drivers/built-in.o: In function `i2c_reg_write':
stmpe-i2c.c:(.text+0xf45ab): undefined reference to
`i2c_smbus_write_byte_data'
drivers/built-in.o: In function `i2c_reg_read':
stmpe-i2c.c:(.text+0xf45d4): undefined reference to `i2c_smbus_read_byte_data'
drivers/built-in.o: In function `stmpe_init':
stmpe-i2c.c:(.init.text+0xaf22): undefined reference to `i2c_register_driver'
drivers/built-in.o: In function `stmpe_exit':
stmpe-i2c.c:(.exit.text+0x5e5): undefined reference to `i2c_del_driver'

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>