arch: mach-imx: imx8m: add pwm ctrl registers fields defines

Add pwm control registers fields defines into imx-regs.h:

 - prescaler
 - dozeen
 - waiten
 - dbgen
 - clksrc_ipg_high
 - clksrc_ipg, en field

References:
 - iMX8MMRM.pdf p 3884

Signed-off-by: Tommaso Merciai <tommaso.merciai@amarulasolutions.com>