driver: net: fsl-mc: Update calculation of MC RAM

Since the reserved RAM is tracked by gd->arch.resv_ram, calculation
of MC memory blocks can be simplified. The MC RAM is guaranteed to be
aligned by the reservation process.

Signed-off-by: York Sun <york.sun@nxp.com>
CC: Priyanka Jain <priyanka.jain@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>