xtensa: io: remove dummy relaxed accessor macros for reads

These are now defined by asm-generic/io.h, so we don't need the private
definitions anymore.

Cc: Chris Zankel <chris@zankel.net>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>