RISC-V: Define MAXPHYSMEM_1GB only for RV32

MAXPHYSMEM_1GB option was added for RV32 because RV32 only supports 1GB
of maximum physical memory. This lead to few compilation errors reported
by kernel test robot which created the following configuration combination
which are not useful but can be configured.

1. MAXPHYSMEM_1GB & RV64
2, MAXPHYSMEM_2GB & RV32

Fix this by restricting MAXPHYSMEM_1GB for RV32 and MAXPHYSMEM_2GB only for
RV64.

Fixes: e5577937

 ("RISC-V: Fix maximum allowed phsyical memory for RV32")
Cc: stable@vger.kernel.org
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>