sh: Tidy up SH-4A boot_cpu_data.flags probing.

This tidies up the boot_cpu_data.flags probing on SH-4A. All of them have
a few things in common, which we can blindly set, rather than having each
subtype have to set the same flags. We can also make assumptions about
cache ways and the validity of PTEA, so this also kills off CPU_HAS_PTEA
as a config option. There was also a bug in the FPU probing, which is now
tidied up.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>