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

Change-Id: I4fa8c25bcd9b7823a11018817a4d17e3357ae681
Signed-off-by: Chandler Zhang <chazhang@nvidia.com>
Reviewed-on: http://git-master/r/211599
GVS: Gerrit_Virtual_Submit
Reviewed-by: Krishna Reddy <vdumpa@nvidia.com>
defconfig: optimize memcpy for cortex-A15