doc: README.fuse: Add an example on how to use the fuse API on mx6q

When using the fuse API in U-boot user must calculate the 'bank' and 'word'
values.

Provide a real example on how to calculate such values for the mx6q.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>