fbdev: arm64 use __raw I/O memory api

Since arm64 also has __raw I/O accessors, add __aarch64__ in fb.h.
This is a supplement for commmit 981409b2 ("fbdev: arm has __raw
I/O accessors, use them in fb.h").

Signed-off-by: Ji Zhang <ji.zhang@mediatek.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>