m32r: Simplify ei_handler code

Simplify and clean up messy ei_handler code in arch/m32r/kernel/entry.S.
- Remove ifdef's for CONFIG_CHIP_* configulations.
- Rearrange the M32700 workaround code.
- Remove the messy platform-dependent interrupt check routines and
  consolidate them to common INT0/INT1/INT2 check routines for all
  platforms with cascaded interrupt controllers.

Signed-off-by: Hitoshi Yamamoto <hitoshiy@linux-m32r.org>
Signed-off-by: Hirokazu Takata <takata@linux-m32r.org>