powerpc/BSR: add 4096 byte BSR size

Add a 4096 byte BSR size which will be used on new machines.  Also, remove
the warning when we run into an unknown size, as this can spam the kernel
log excessively.

Signed-off-by: Sonny Rao <sonnyrao@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>