arm64/fixmap: make notes of fixed_addresses more precisely

These 'compile-time allocated' memory buffers can occupy more than one
page and each enum increment is page-sized. So improve the note about it.

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/1596460720-19243-1-git-send-email-kernelfans@gmail.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>