sparc32: drop LEON hack for ASI_M_MMUREGS

All users of MMUREGS ASI is now LEON/SUN aware,
so this is no longer required.

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