[ARM] fix AT91 include loops

AT91 has one include loop in its header files:

  include/asm-arm/io.h <- include/asm-arm/arch-at91/io.h <-
   include/asm-arm/io.h

Circular include dependencies are dangerous since they can result in
inconsistent definitions being provided to other code, especially if
'#ifndef' constructs are used.

Solve this by removing the offending includes.  Built tested using my
AT91 configuration.

Acked-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>