[MIPS] Let gcc align 'struct pt_regs' on 8 bytes boundary

The stack pointer in MIPS/gcc should always 8 bytes aligned on
entry to any routines. Therefore pt_regs structure must be
aligned to 8-byte boundary too.

Instead of creating dummy fields to achieve this alignment, this
patch let gcc doing it. Therefore 'smtc_pad' field can be safely
removed.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>