riscv: try to allocate crashkern region from 32bit addressible memory

When allocating crash kernel region without explicitly specifying its
base address/size, memblock_phys_alloc_range will attempt to allocate
memory top to bottom (memblock.bottom_up is false), so the crash
kernel region will end up in highmem on 64bit systems. This way
swiotlb can't work on the crash kernel, since there won't be any
32bit addressible memory available for the bounce buffers.

Try to allocate 32bit addressible memory if available, for the
crash kernel by restricting the top search address to be less
than SZ_4G. If that fails fallback to the previous behavior.

I tested this on HiFive Unmatched where the pci-e controller needs
swiotlb to work, with this patch it's possible to access the pci-e
controller on crash kernel and mount the rootfs from the nvme.

Signed-off-by: Nick Kossifidis <mick@ics.forth.gr>
Fixes: e53d2818 ("RISC-V: Add kdump support")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>