powerpc: mpc8xx: redistribute data in CPM dpram

Some malloc memory is needed at startup for DM model.
Lets reorganise the use of the CPM dpram.

The MPC866/885 dpram, we have 8kbytes dual port RAM, which is usable as:
IMMR + 0x2000..0x2800: BD/Data/Microcode
IMMR + 0x2800..0x2e00: BD/Data
IMMR + 0x2e00..0x3800: BD/Data/Microcode
IMMR + 0x3800..0x3a00: BD/Data
IMMR + 0x3a00..0x3c00: BD/Data/Microcode
IMMR + 0x3c00..0x4000: Parameters for the Peripheral Controllers

Lets reallocate all BDs in the 3800..3a00 area and give the full
2800..2e00 for dynamic RAM allocation including global data

That way, the microcode areas remain available if needed one day.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>