powerpc/mm/book3s64: Fix build error with SPARSEMEM disabled

With CONFIG_SPARSEMEM disabled the below kernel build error is observed.

 arch/powerpc/mm/init_64.c:477:38: error: use of undeclared identifier 'SECTION_SIZE_BITS'

CONFIG_MEMORY_HOTPLUG depends on CONFIG_SPARSEMEM and it is more clear
to describe the code dependency in terms of MEMORY_HOTPLUG. Outside
memory hotplug the kernel uses memory_block_size for kernel directmap.
Instead of depending on SECTION_SIZE_BITS to compute the direct map
page size, add a new #define which defaults to 16M(same as existing
SECTION_SIZE)

Fixes: 4d157211 ("powerpc/mm: Cleanup memory block size probing")
Signed-off-by: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308251532.k9PpWEAD-lkp@intel.com/


Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230828074658.59553-1-aneesh.kumar@linux.ibm.com