mpc8569mds: fix CONFIG_ENV_SIZE

CONFIG_ENV_SIZE of MPC8569MDS was wrongly set to CONFIG_ENV_SECT_SIZE which
is 128KB, so it took longer time to do crc32 calculation for ENV than it should
do. It causes the bootup for MPC8569MDS significantly slow. This patch fixs it
to 0x2000(8KB), also fix the comment for CONFIG_ENV_SECT_SIZE to correct size.

Signed-off-by: Kai.Jiang <Kai.Jiang@freescale.com>
Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>