RISC-V: MAXPHYSMEM_2GB doesn't depend on CMODEL_MEDLOW

For non-relocatable kernels we need to be able to link the kernel at
approximately PAGE_OFFSET, thus requiring medany (as medlow requires the
code to be linked within 2GiB of 0).  The inverse doesn't apply, though:
since medany code can be linked anywhere it's fine to link it close to
0, so we can support the smaller memory config.

Fixes: de5f4b8f ("RISC-V: Define MAXPHYSMEM_1GB only for RV32")
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>