[ARM] 5306/1: pxa: fix build error on CM-X270

Fix build for CM-X2XX with CONFIG_RTC_DRV_V3020 unset:

CC      arch/arm/mach-pxa/cm-x270.o
/mnt/sdb1/git/linux-2.6-arm/arch/arm/mach-pxa/cm-x270.c: In function 'cmx270_init':
/mnt/sdb1/git/linux-2.6-arm/arch/arm/mach-pxa/cm-x270.c:338: error: implicit declaration of function 'cmx270_init_rtc'
make[2]: *** [arch/arm/mach-pxa/cm-x270.o] Error 1

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>