ARM: Update mach-types to fix mxs build breakage

Add entry m28evk to fix the following mxs build breakage.

  CHK     include/generated/compile.h
  CC      arch/arm/mach-mxs/clock-mx28.o
arch/arm/mach-mxs/clock-mx28.c: In function 'clk_misc_init':
arch/arm/mach-mxs/clock-mx28.c:748: error: implicit declaration of
function 'machine_is_m28evk'
make[1]: *** [arch/arm/mach-mxs/clock-mx28.o] Error 1
make: *** [arch/arm/mach-mxs] Error 2

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>