powerpc/book3e-64: Use hardcoded mttmr opcode

This preserves the ability to build using older binutils (reportedly <=
2.22).

Fixes: 6becef7e

 ("powerpc/mpc85xx: Add CPU hotplug support for E6500")
Signed-off-by: Scott Wood <oss@buserror.net>
Cc: chenhui.zhao@freescale.com
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>