board/ls2080ardb: Add mcmemsize variable in default env

For most of ls2080ardb use-cases, mc private DRAM block is required
to be of 1.75GB.

Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>
[YS: this reservation needs to be reduced if memory is not enough]
Reviewed-by: York Sun <york.sun@nxp.com>