Another case of replacing hard coded constants, this time
referring to the definition in the virt machine's memmap.

Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: NMichael Clark <mjc@sifive.com>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NAlistair Francis <alistair.francis@wdc.com>