powerpc/85xx: Update P2020DS default env settings

Read MAC address from EEPROM. Add hwconfig settings.

Modified the default othbootargs to include the cache-sram-size
parameter. This parameter is needed as the L2 as SRAM is ON by
default in the P2020RDB kernel and used by the Gianfar driver.

Also cleanup some of the boot commands.

Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Zhao Chenhui <b35336@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>