sh: Convert Dreamcast support from hw_interrupt_type to irq_chip

Switch the dreamcast IRQ code over to the irq_chip way of doing things,
so that we can set GENERIC_HARDIRQS_NO__DO_IRQ for all SuperH boards.

Also, whilst I'm here change some things to make checkpatch.pl happy:
	- Indent with tabs, not with spaces
	- Include <linux/io.h>, not <asm/io.h>
	- Fix the multi-line comment style
	- Fix some typos in the comments

Tested-by: Adrian McMenamin <adrian@newgolddream.dyndns.info>
Signed-off-by: Matt Fleming <mjf@gentoo.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>