hw/intc/i8259: Implement legacy LTIM Edge/Level Bank Select

hw/intc/i8259: Implement legacy LTIM Edge/Level Bank Select

Back in the mists of time, before EISA came along and required per-pin
level control in the ELCR register, the i8259 had a single chip-wide
level-mode control in bit 3 of ICW1.

Even in the PIIX3 datasheet from 1996 this is documented as 'This bit is
disabled', but apparently MorphOS is using it in the version of the
i8259 which is in the Pegasos2 board as part of the VT8231 chipset.

It's easy enough to implement, and I think it's harmless enough to do so
unconditionally.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
[balaton: updated commit message as asked by author]
Tested-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <3f09b2dd109d19851d786047ad5c2ff459c90cd7.1678188711.git.balaton@eik.bme.hu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>