iio: Add t5403 barometric pressure sensor driver

16-bit pressure and temperature sensor

the chip can do I2C and SPI, only the I2C interface is supported
by the driver at the moment

datasheet: http://www.epcos.com/inf/57/ds/T5400.pdf
application note: http://www.epcos.com/blob/993154/download/1/t5403-applicationnote.pdf

an out-of-tree driver targetting the input subsystem is at
https://github.com/unixphere/t5400, it was rejected here:
http://comments.gmane.org/gmane.linux.kernel.input/28107



v2: (thanks Hartmut Knaack)
* fix MODE_HIGH, equals 2
* check INT_TIME mask in write_raw()

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Stefan Nilsson <stefan.nilsson@unixphere.com>
Reviewed-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>