microblaze: Remove _start symbol handling at U-Boot start

microblaze: Remove _start symbol handling at U-Boot start

Right now U-Boot runs all the time from the same address where it is loaded
but going to full relocation code starting address doesn't need to be fixed
and can be simply discovered from reading PC register. That's why use r20
to get PC address and subtract offset from the beginning to get starting
address.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/044b727c33dfbe662f68512d0da0775a4805f360.1655299267.git.michal.simek@amd.com