target/ppc: merge ppc_vsr_t and ppc_avr_t union types

target/ppc: merge ppc_vsr_t and ppc_avr_t union types

Since the VSX registers are actually a superset of the VMX registers then they
can be represented by the same type. Merge ppc_avr_t into ppc_vsr_t and change
ppc_avr_t to be a simple typedef alias.

Note that due to a difference in the naming of the float32 member between
ppc_avr_t and ppc_vsr_t, references to the ppc_avr_t f member must be replaced
with f32 instead.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>