riscv: mm: Define MAX_POSSIBLE_PHYSMEM_BITS for zsmalloc

This definition is used by zsmalloc to optimize memory allocation. On
riscv64, it is the same as MAX_PHYSMEM_BITS from asm/sparsemem.h, but
that definition depends on CONFIG_SPARSEMEM. The correct definition is
already provided for riscv32.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Link: https://lore.kernel.org/r/20251015233327.3885003-1-samuel.holland@sifive.com


Signed-off-by: Paul Walmsley <pjw@kernel.org>