powerpc: mpc8xx: Reorganise init RAM

Using SMC relocation microcode patch or USB-SOF microcode patch
will disable DPRAM memory from 0x2000 to 0x2400 and from 0x2f00
to 0x3000.

At the time being, init RAM is setup to use 0x2800-0x2e00, but
the stack pointer goes beyond 0x2800 and even beyond 0x2400.

For the time being we are not going to use any microcode patch
that uses memory about 0x3000, so reorganise setup to use:
- 0x2800 - 0x2e00 for init malloc and global data and CPM buffers
- 0x3000 - 0x3c00 for init stack

For more details about CPM dual port ram, see
commit b1d62424 ("powerpc: mpc8xx: redistribute data in CPM dpram")

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>