arm64: fixmap: make FIX_TEXT_POKE0 permanent

The FIX_TEXT_POKE0 is currently at the end of the temporary fixmap
slots, despite the fact that it can be used at any point during runtime
(e.g. for poking the text of loaded modules), and thus should be a
permanent fixmap slot (as is the case on arm and x86).

This patch moves FIX_TEXT_POKE0 into the set of permanent fixmap slots.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>