[U-Boot] V2 remove omap3 compiler warning wrt v7_flush_dcache_all

[Tom Rix]

This is against the arm/next branch. 

This fixes a problem with compile warnings for all OMAP3 targets. 

As per feedback, v7_flush_dcache_all was renamed to the generic
flush_dcache and moved to the omap3 level in the new file cache_flush.S. 

The original fix of using sys_proto.h is still used to fix the warning
for for get_device_type.

compile tested on MAKEALL arm. 
run tested on zoom1.