ppc4xx: Add CONFIG_4xx_DCACHE compile options to enable cached SDRAM

This patch adds the CONFIG_4xx_DCACHE options to some SDRAM init files
and to the Sequoia TLB init code. Now the cache can be enabled on 44x
boards by defining CONFIG_4xx_DCACHE in the board config file. This
option will disappear, when more boards use is successfully and no
more known problems exist.

This is tested successfully on Sequoia and Katmai. The only problem that
needs to be fixed is, that USB is not working on Sequoia right now, since
it will need some cache handling code too, similar to the 4xx EMAC driver.

Signed-off-by: Stefan Roese <sr@denx.de>