RISC-V: asm/page.h: fix spelling mistake "CONFIG_64BITS" -> "CONFIG_64BIT"

There is no CONFIG_64BITS Kconfig macro.
Please see arch/riscv/Kconfig for details, e.g.

  linux$ git grep -HnA 1 "config 64BIT" arch/riscv/Kconfig
  arch/riscv/Kconfig:6:config 64BIT
  arch/riscv/Kconfig-7-        bool
Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/asm/page.h

@@ -80,7 +80,7 @@ typedef struct page *pgtable_t;
 #define __pgd(x)	((pgd_t) { (x) })
 #define __pgprot(x)	((pgprot_t) { (x) })
 
-#ifdef CONFIG_64BITS
+#ifdef CONFIG_64BIT
 #define PTE_FMT "%016lx"
 #else
 #define PTE_FMT "%08lx"