[ARM] pxa: don't register lpd270 cpld_irq sysdev if !lpd270

Don't register the LPD270 cpld_irq system device when we're not running
on a LPD270 machine - "cpld_irq" is also registered (separately) by
Lubbock and Mainstone.

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