ARM: 6271/1: Introduce *_relaxed() I/O accessors

This patch introduces readl*_relaxed()/write*_relaxed() as the main I/O
accessors (when __mem_pci is defined). The standard read*()/write*()
macros are now based on the relaxed accessors.

This patch is in preparation for a subsequent patch which adds barriers
to the I/O accessors.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>