microblaze: Do not initialized regs->r1 twice in ELF_PLAT_INIT

Fix ELF_PLAT_INIT macro which initialized r1 twice which
ends in compilation warning.

Warning log:
fs/binfmt_elf.c: In function 'load_elf_binary':
fs/binfmt_elf.c:981:2: warning: operation on 'regs->r1' may be undefined [-Wsequence-point]
  CC      fs/dcookies.o

Signed-off-by: Michal Simek <michal.simek@xilinx.com>