ARM: riscpc: add ecard quirk for Atomwide 3port serial card

Atomwide 3port serial cards seem to leave their interrupts active when
exiting RISC OS, resulting in an interrupt storm during boot, and the
expansion card interrupt being disabled.  Avoid this by manually
disabling the interrupt on each serial port via a custom quirk function.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>