iio: pressure: mpl115: support MPL115A1

mpl115 driver currently supports i2c interface (MPL115A2).
There is also SPI version (MPL115A1).  The difference between them
is only physical transport so we can easily support both while sharing
most of the code.

Split the driver into a core support module and one module each for I2C
and SPI support.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald <pmeerw@pmeerw.net>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>