x86/efi: Update comment regarding required phys mapped EFI services

Commit 3f4a7836e331 ("x86/efi: Rip out phys_efi_get_time()") left
set_virtual_address_map as the only runtime service needed with a
phys mapping but missed to update the preceding comment. Fix that.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>