microblaze: Optimize register usage in relocate_code

There are additional operations which can be done simpler that's why
improve logic around relocation address r7 handling and _start symbol.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/c8b60f72f1605c2ba6b4b7be1893d7e6ec3d8597.1655299267.git.michal.simek@amd.com
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