arch: arm: lib: optimize memcpy for cortex-A15

The LDRD/STRD instruction is faster than LDM/STM on Cortex-A15.
Also optimized preload cache size for Cortex-A15.

Added USE_LDRDSTRD_OVER_LDMSTM to turn on LDRD/STRD optimization.
Added ARM_PLD_SIZE, default 32. Should set to 64 for Cortex-A15.

Bug 1185248

Signed-off-by: Chandler Zhang <chazhang@nvidia.com>
Reviewed-by: Krishna Reddy <vdumpa@nvidia.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
hells_defconfig: Enable memcpy optimizations for cortex A-15