socfpga: arria10: Improve bitstream loading speed

Apply some optimizations to speed up bitstream loading
(both for full and split periph/core bitstreams):

 * Change the size of the first fs read, so that all the subsequent
   reads are aligned to a specific value (called MAX_FIRST_LOAD_SIZE).
   This value was chosen so that in subsequent reads the fat fs driver
   doesn't have to allocate a temporary buffer in get_contents
   (assuming 8KiB clusters).

 * Change the buffer size to a larger value when reading to ddr
   (but not too large, because large transfers cause a stack overflow
   in the dwmmc driver).

Signed-off-by: Paweł Anikiel <pan@semihalf.com>
Reviewed-by: Simon Glass <sjg@chromium.org>