mpc8xx: Expose show_regs()

To match the other PowerPC platforms the function show_regs() must not
be marked static but instead be an exposed global function.

Cc: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Christophe Leroy <christophe.leroy@c-s.fr>