platform/mulle: Use symbolic bit names in flash.c

Replaced magic numbers by their symbolic equivalents from MK60DZ10.h.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Do not power on Vperiph from flash_init.

Vperiph should be controlled separately using Mulle power supply API
and is already initialized there.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Clean up clock configuration.

System, Bus, FlexBus, and Flash clock frequencies are determined at
compile time by F_CPU together with the divider settings in
config-clocks.h

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>