ARM: CPU: arm926ejs: Consolidate cache routines to common file

Four different boards had different options for enabling cache
that were virtually all the same.  This consolidates these
common functions into arch/arm/cpu/arm926ejs/cache.c

This also has the positive side-effect of enabling cache on
the Davinci (da850) boards.

Signed-off-by: Adam Ford <aford173@gmail.com>
[trini: Add mach-at91 to the list of consolidations]
Signed-off-by: Tom Rini <trini@konsulko.com>