dm6446: disable cache usage due to coherency issues

dm6446: disable cache usage due to coherency issues

there are cache coherency issues when using the DAVINCI Ethernet driver,
hence caches cant be used for dm6446 u-boot. As per new cache management
framework,if the caches are not used in u-boot, it needs to be explicitly
indicated through macros in config file. CACHE disable is  indicated by
the following macro definitions in config file,

1. CONFIG_SYS_ICACHE_OFF
2. CONFIG_SYS_DCACHE_OFF
3. CONFIG_SYS_L2CACHE_OFF

Signed-off-by: Nagabhushana Netagunte <nagabhushana.netagunte@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>