Added include/asm-ppc/ppc4xx-isram.h and moved internal SRAM and
L2 cache DCRs from ppc440.h to this new header.

Also converted these DCR defines from lowercase to uppercase and
modified referencing modules to use them.

Signed-off-by: Dave Mitchell <dmitch71@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>