rtc-pxa: fix build failure

Fix these build errors:

  CC      drivers/rtc/rtc-pxa.o
drivers/rtc/rtc-pxa.c: In function `pxa_rtc_init':
drivers/rtc/rtc-pxa.c:472: error: implicit declaration of function `cpu_is_pxa27x'
drivers/rtc/rtc-pxa.c:472: error: implicit declaration of function `cpu_is_pxa3xx'

Signed-off-by: Antonio Ospite <ao2@openezx.org>
Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>