ARM: 7951/1: uaccess: use CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS

Now that we select HAVE_EFFICIENT_UNALIGNED_ACCESS for ARMv6+ CPUs,
replace the __LINUX_ARM_ARCH__ check in uaccess.h with the new symbol.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>