arm64: bcm2835: Add missing dependency on MFD_CORE.

commit 7a9b6be9 upstream.

When adding the MFD dependency for power domains and WDT in bcm2835, I
added it only on the arm32 side and missed it for arm64.

Fixes: 5e6acc3e

 ("bcm2835-pm: Move bcm2835-watchdog's DT probe to an MFD.")
Signed-off-by: Eric Anholt <eric@anholt.net>
Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>