!449 at91sam9260平台增加mtdnand驱动用于支持uffs文件系统

* update bsp/at91sam9260/drivers/at91_nand.c.
* update bsp/at91sam9260/drivers/at91_nand.c.
* update bsp/at91sam9260/drivers/at91_nand.h.
* update bsp/at91sam9260/drivers/at91_nand.c.
* update bsp/at91sam9260/drivers/at91_nand.c.
* 添加at91sam9260下mtd nand flash驱动

Author: brightsally

bsp/at91sam9260/drivers/SConscript

@@ -23,6 +23,8 @@ if GetDepend('RT_USING_LWIP'):
 if GetDepend('RT_USING_I2C') and GetDepend('RT_USING_I2C_BITOPS'):
 	src += ['at91_i2c_gpio.c']
 
+if GetDepend('RT_USING_MTD_NAND'):
+    src += ['at91_nand.c']
 
 CPPPATH = [cwd]