.gitignore: Add *.mulle for ignoring Mulle binaries.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Add LIS3DH accelerometer driver.

This driver is split into two parts, one platform specific part
in lis3dh-arch.c, and one platform independent part in lis3dh.c

Both parts currently live inside the platform/mulle/dev directory
but it should be possible to move the lis3dh.[ch] to a more general
directory.

Included are two examples for reading the data from the accelerometer.

- examples/lis3dh-poll is the minimal example for initializing the
   hardware and to read live samples.

- examples/lis3dh-fifo is an example for using the builtin FIFO
   for reading multiple samples in quick succession. This example
   also computes some statistics on the accelerometer data.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>