BCM270x: Enable bcm2835_wdt and bcm2835-rng

Change the kconfig dependency to make bcm2835_wdt and bcm2835-rng
available on ARCH_BCM2708 and ARCH_BCM2709.
Enable them as loadable modules in bcmrpi_defconfig and
bcm2709_defconfig.

There is a commit in linux-next that will move restart/pm_power_off
to bcm2835_wdt for ARCH_BCM2835. This will not affect ARCH_BCM270x
since arm_pm_restart (.restart = bcm2708_restart) and
pm_power_off (=bcm2708_power_off) is set in
arch/arm/mach-bcm270X/bcm270X.c and will take presedence.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>