imx: re-work of PWM, add i.MX21 support

* Kconfig enables now HAVE_PWM (this enables in turn the selection of
  CONFIG_BACKLIGHT_PWM)
* changes CONFIG_ARCH_MXyy to CONFIG_MACH_MXyy
* fix some register names to match those of the reference manual
* write a stub code so that the PWM can be used to program the
  LCD backlight
* convert from #ifdef CONFIG_ARCH_MXxx to cpu_is_mxXX()
* remove unneeded defines and fix 80-column "violations" of checkpatch.pl

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>