powerpc/mm: Export mmu_kernel_ssize and mmu_linear_psize

Export mmu_kernel_ssize and mmu_linear_psize.  These are needed by the cxl
driver which has it's own MMU.  To setup the MMU cxl needs access to these.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>