Remove 'epit' from generic i.MX platform spec

Remove the 'epit' variable from the generic imx31 and imx53 specification,
and only add it to base-hw specific i.MX specs. Thereby the EPIT timer
library gets build for base-hw only.
Moreover, fix some const-ness issues in the platform_timer implementation
for the EPIT timer.
Fixes #688.