sparc: introduce CONFIG_BITS

CONFIG_BITS is set to 32 for sparc32
and 64 for sparc64.

This allow us to use this symbol in for example header files
to ease unification of sparc32 and sparc64.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>