powerpc/mpc85xx: Remove some dead code

U-Boot does not have system calls (the services it exposes to
standalone commands use a different mechanism), so the syscall handler
is dead code.  It's also broken code, as it assumes it is located at
0xc00 -- while even before the patch to stop relocating exception
vectors to 0, U-Boot had the syscall at 0x900.

The critical and machine check return paths are never called -- the
regular exception return path is used instead, which works because
xSRR0/1 have already been saved and can be restored via the regular
SRR0/1 (we don't care too much in U-Boot about taking a critical/mcheck
inside another exception prolog/epilog).

Also remove a few other small unused functions.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>