powerpc: Modify some ppc_asm.h macros to accomodate 64-bits Book3E

The way I intend to use tophys/tovirt on 64-bit BookE is different
from the "trick" that we currently play for 32-bit BookE so change
the condition of definition of these macros to make it so.

Also, make sure we only use rfid and mtmsrd instead of rfi and mtmsr
for 64-bit server processors, not all 64-bit processors.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Kumar Gala <galak@kernel.crashing.org>