microblaze: Save and restore r3/r4 in SAVE/RESTORE_REGS macros

Save and restore R3/R4 registers in macros. This change
help to cleanup entry.S.

In ret_from_trap function we are saving returning value from
syscall to pt_regs on stack that's why we don't need to save and
restore these values before kernel functions (schedule, do_signal).

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