platform: sifive: fu540: allow sv32 as an mmu-type

There has already been a commit to master which added 32-bit specific
fdt/payload addresses for the sifive/fu540 platform [0]. This commit
introduces another change for using sifive/fu540 as a 32-bit platform.
On 32-bit platforms, cores with the SV32 MMU type should not be
disabled. For this reason, this commit also allows using this MMU type
on the sifive/fu540 platform.

Alternatively it would also be possible to only allow SV39 and SV48 if
`__riscv_xlen == 64` and SV32 if `__riscv_xlen == 32`. Removing the
check entirely would also be an option.

[0]: 66fb729a



Signed-off-by: Sören Tempel <tempel@uni-bremen.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>