[ARM] 4369/1: AT91: Fix circular dependency in header files

Resolve the circular dependency in the AT91 header files (io.h and
hardware.h) by moving the at91_sys_read() and at91_sys_write() functions
to io.h

Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>