pwm: fsl-ftm: Select REGMAP_MMIO

Commit 42fa98a9

 ("pwm: fsl-ftm: Convert to direct regmap API usage")
introduced the following error when REGMAP_MMIO=n:

drivers/built-in.o: In function `fsl_pwm_probe':
>> pwm-fsl-ftm.c:(.text+0xd7d7): undefined reference to `devm_regmap_init_mmio_clk'

Select select REGMAP_MMIO in order to fix this error.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>