arm: clocksource: mvebu: Use the main timer as clock source from DT

This commit:
  573145f0
  clocksource: armada-370-xp: Use CLOCKSOURCE_OF_DECLARE

replaced a call to the driver's timer initialization by a call to
clocksource_of_init(). However, it failed to select CONFIG_CLKSRC_OF.

Fix this by selecting CONFIG_CLKSRC_OF for Armada370/XP machines.
Without this change the kernel is stuck at: 'Calibrating delay loop...'.

Signed-off-by: Jean Pihet <jean.pihet@linaro.org>
Acked-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>