ppc4xx: Add Canyonlands NAND booting support

460EX doesn't support a fixed bootstrap option to boot from 512 byte page
NAND devices. The only bootstrap option for NAND booting is option F for
2k page devices. So to boot from a 512 bype page device, the I2C bootstrap
EEPROM needs to be programmed accordingly.

This patch adds basic NAND booting support for the AMCC Canyonlands aval
board and also adds support to the "bootstrap" command, to enable NAND
booting I2C setting.

Tested with 512 byte page NAND device (32MByte) on Canyonlands.

Signed-off-by: Stefan Roese <sr@denx.de>