cva6: u-boot improvements

• cva6: Reserve opensbi memory in dts That memory is occupied by opensbi. Actually only part of it, and if we can use data provided by opensbi about actual memory usage, we can get more free memory. But for now hardcoded values will do.

  Linux used to reserve first 2MB of memory on 64bit and 4MB on 32bit, but that changed in newer versions. This fix makes newer Linux avoid touching opensbi data and triggering PMP.

• cva6: Increase binary size

  New kernel versions are bigger, so increase the size

• riscv: Fix build against binutils 2.38

  Cherry-picked from upstream u-boot: https://github.com/u-boot/u-boot/commit/1dde977518f13824b847e23275001191139bc384