sparc32: refactor cpu_idle()

With the removal of sun4c we can use the same cpu_idle()
implementation on UP and SMP.
This also fix it so we use the same version independent on LEON
enabled or not.

V2: Fixed whitespace issue pointed out by Josip Rodin.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Josip Rodin <joy@entuzijast.net>
Cc: Daniel Hellstrom <daniel@gaisler.com>
Cc: Konrad Eisele <konrad@gaisler.com>