linux_compat: remove cpu_relax() define

The macro cpu_relax() is defined by several headers in different
ways.

arch/{arm,avr32,mips}/include/asm/processor.h defines it as follows:
  #define cpu_relax()	barrier()

On the other hand, include/linux/compat.h defines it as follows:
  #define cpu_relax() do {} while (0)

If both headers are included from the same source file, the warning
  warning: "cpu_relax" redefined [enabled by default]
is displayed.

It effectively makes it impossible to include <linux/compat.h>
from some sources.  Drop the latter.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>