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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Paul Walmsley <[email protected]>

Author: SiFiveHolland

arch/riscv/include/asm/pgtable-64.h

@@ -69,6 +69,8 @@ typedef struct {
 
 #define PTRS_PER_PMD    (PAGE_SIZE / sizeof(pmd_t))
 
+#define MAX_POSSIBLE_PHYSMEM_BITS 56
+
 /*
  * rv64 PTE format:
  * | 63 | 62 61 | 60 54 | 53  10 | 9             8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0