microblaze: ftrace: Pass the first calling instruction for dynamic ftrace

Selftest for dynamic ftrace requres to pass address of the first
calling instruction because hash function is calculated from it.

ftrace_update_ftrace_func setups pointer to function which is called
in _mcount function. trace_selftest is not aware about instruction
size (for microblaze 8 - imm and addik) and that's why we have
to pass in r5 address of imm not addik which is in r15.12

For more info look at ftrace_ops_list_func/ftrace.c.

Signed-off-by: Michal Simek <monstr@monstr.eu>