fs: btrfs: Crossport btrfs_read_sys_array() and btrfs_read_chunk_tree()

These two functions play a big role in btrfs bootstrap.

The following function is removed:
- Seed device support

Although in theory we can still support multiple devices, we don't have
a facility in U-Boot to do device scan without opening them.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>