arm64: kernel: fix style issues in sleep.S

This fixes a number of style issues in sleep.S. No functional changes are
intended:
- replace absolute literal references with relative references in
  __cpu_suspend_enter(), which executes from its virtual address
- replace explicit lr assignment plus branch with bl in cpu_resume(), which
  aligns it with stext() and secondary_startup()
- don't export _cpu_resume()
- use adr_l for mpidr_hash reference, and fix the incorrect accompanying
  comment, which has been out of date since commit cabe1c81

 ("arm64:
  Change cpu_resume() to enable mmu early then access sleep_sp by va")
- replace leading spaces with tabs, and add a bit of whitespace for
  readability

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>