ppc4xx: Changed 460EX/GT OCM TLB and internal SRAM initialization

Expanded OCM TLB to allow access to 64K OCM as well as 256K of
internal SRAM.

Adjusted internal SRAM initialization to match updated user
manual recommendation.

OCM & ISRAM are now mapped as follows:
        physical        virtual         size
ISRAM   0x4_0000_0000   0xE300_0000     256k
OCM     0x4_0004_0000   0xE304_0000     64k

A single TLB was used for this mapping.

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