drivers/clocksource: timer-fsl-ftm: use common header for FlexTimer #defines

Common #defines have been moved to "linux/fsl/ftm.h". Thus making use of
this file.
Also FTM_SC_CLK_SHIFT has been renamed to FTM_SC_CLK_MASK_SHIFT.

Reviewed-by: Esben Haabendal <esben@haabendal.dk>
Signed-off-by: Patrick Havelange <patrick.havelange@essensium.com>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>