arm: atmel: add nand trimffs subcommand for at91sam9n12 and at91sam9x5

as the at91sam9n12 and at91sam9x5 soc support PMECC, when use u-boot
to flash the rootfs, in order to avoid flash one sector with all 0xff
into NAND, so use nand trimffs subcommand to avoid it

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