atmel: at91sam9x5: add spi flash boot support

Add at91sam9x5 series spi flash boot support

Using at91sam9x5ek_spiflash to configure, then it can boot from at25df321
serial flash

SPI mater work in 30Mhz speed, while not 1Mhz speed. This will base on
atmel_spi patch, or else, it will occur receive overrun

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>