ARM:asm:io.h use static inline

When compiling u-boot with W=1 the extern inline void for
read* is likely causing the most noise. gcc / clang will
warn there is never a actual declaration for these functions.
Instead of declaring these extern make them static inline so
it is actually declared.

cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>