sh/mm: Make pmd_t similar to pte_t

Just like 64bit pte_t, have a low/high split in pmd_t.
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221022114424.645657294%40infradead.org

arch/sh/include/asm/pgtable-3level.h

@@ -28,9 +28,15 @@
 #define pmd_ERROR(e) \
 	printk("%s:%d: bad pmd %016llx.\n", __FILE__, __LINE__, pmd_val(e))
 
-typedef struct { unsigned long long pmd; } pmd_t;
+typedef struct {
+	struct {
+		unsigned long pmd_low;
+		unsigned long pmd_high;
+	};
+	unsigned long long pmd;
+} pmd_t;
 #define pmd_val(x)	((x).pmd)
-#define __pmd(x)	((pmd_t) { (x) } )
+#define __pmd(x)	((pmd_t) { .pmd = (x) } )
 
 static inline pmd_t *pud_pgtable(pud_t pud)
 {