mips: sgi-ip27: switch from DISCONTIGMEM to SPARSEMEM

mips: sgi-ip27: switch from DISCONTIGMEM to SPARSEMEM

The memory initialization of SGI-IP27 is already half-way to support
SPARSEMEM. It only had free_bootmem_with_active_regions() left-overs
interfering with sparse_memory_present_with_active_regions().

Replace these calls with simpler memblocks_present() call in prom_meminit()
and adjust arch/mips/Kconfig to enable SPARSEMEM and SPARSEMEM_EXTREME for
SGI-IP27.

Co-developed-by: Thomas Bogendoerfer <[email protected]>
Signed-off-by: Thomas Bogendoerfer <[email protected]>
Signed-off-by: Mike Rapoport <[email protected]>
Signed-off-by: Paul Burton <[email protected]>
Cc: [email protected]
Cc: [email protected]