powerpc/math-emu: Fix load/store indexed emulation

Load/store indexed instructions where the index register RA=R0, such
as "lfdx f1,0,r3", are not illegal.

Load/store indexed with update instructions where the index register
RA=R0, such as "lfdux f1,0,r3", are invalid, and, to be consistent
with existing math-emu behavior for other invalid instruction forms,
will signal as illegal.

Signed-off-by: James Yang <James.Yang@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>