ppc4xx: Add bootstrap command for AMCC Sequoia (440EPx) eval board

This patch adds a board command to configure the I2C bootstrap EEPROM
values. Right now 533 and 667MHz are supported for booting either via NOR
or NAND FLASH. Here the usage:

=> bootstrap 533 nor        ;to configure the board for 533MHz NOR booting
=> bootstrap 667 nand       ;to configure the board for 667MHz NNAND booting

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