sama5d3x: add 'flush_bootstrap_nand' command to flush bootstrap to nand

This command can flush at91bootstrap or u-boot-spl to nand flash by
U-Boot.

Usage is:
  1) tftp u-boot-spl.bin; run flush_bootstrap_nand;
  2) fatload mmc 0 22000000 u-boot-spl.bin; run flush_bootstrap_nand;

This command will do following work to make binary can boot in nand
flash:
  * Write the binary size with a 32bit word in the offset 0x14
    of the binary. ROM code will check this offset.
  * Add a PMECC header. which is made of 52 times the same 32-bit
    word. The word include the PMECC information.

For more information can refer to Boot Strategies Section in at91sam9x5
datasheet (SAMA5D3 is the same):
  http://www.atmel.com/images/atmel_11055_32-bit-arm926ej-s-
  microcontroller_sam9x35_datasheet.pdf

Signed-off-by: Josh Wu <josh.wu@atmel.com>