ARM: 7687/1: atomics: don't use exclusives for atomic64 read/set with LPAE

To ease page table updates with 64-bit descriptors, CPUs implementing
LPAE are required to implement ldrd/strd as atomic operations.

This patch uses these accessors instead of the exclusive variants when
performing atomic64_{read,set} on LPAE systems.

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