target-sparc: Implement CPUClass::parse_features() for SPARCCPU

Factor cpu_model parsing out of cpu_sparc_find_by_name() by passing
cpu_sparc_find_by_name() the name portion only and calling
CPUClass::parse_features() from cpu_sparc_register() afterwards.

Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Andreas Färber <afaerber@suse.de>