[SPARC64]: Fix sun4v early bootup.

prom_sun4v_name should be "sun4v" not "SUNW,sun4v"

Also, this is too early to make use of the
.sun4v_Xinsn_patch code patching, so just check
things manually.

This gets us at least to prom_init() on Niagara.

Signed-off-by: David S. Miller <davem@davemloft.net>