SmartMatrix.cpp - loadMatrixBuffers - Zero all the pad and clk bits in parallel

Use 2 MOVS instructions instead of 8 BFC instructions
Inside loop, so saves 192 instructions per row
(at 16 rows for a 32x32 panel, and 120 refresh per second this
saves about 368,000 instructions per second)
should mean less overhead
Merge pull request #2 from ecdr/speedup

SmartMatrix.cpp - loadMatrixBuffers - Zero all the pad and clk bits in p...