mfd: atmel-flexcom: Create include file with macros used by DT bindings

This patch defines some macros to be used as value for the
"atmel,flexcom-mode" DT property. This value is then written into
the Operating Mode (OPMODE) bit field of the Flexcom Mode Register.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>