microblaze: Create SYM_ADDR macro to deal with symbols

Symbol handling depends on compilation flags. Right now manual relocation
is used that's why symbols can be referenced just by name and there is no
need to find them out. But when position independent code (PIC) is used
symbols need to be described differently. That's why having one macro
change is easier than changing the whole code.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/d704e9a267c8b536452fb999111dbfbc9d652be5.1655299267.git.michal.simek@amd.com