km-powerpc: define CONFIG_PRAM to protect PHRAM and PNVRAM

km-powerpc: define CONFIG_PRAM to protect PHRAM and PNVRAM

When u-boot initializes the RAM (early in boot) it looks for the "pram"
env variable to know which is area it cannot use. If the "pram" env variable
is not found, the default CONFIG_PRAM value is used.

This value used to be 0 (no protection at all). This patch sets it to a
value that covers PHRAM and PNVRAM that must be protected in our case.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Reviewed-by: York Sun <yorksun@freescale.com>