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>