PM / devfreq: rockchip: add PM_DEVFREQ_EVENT dependency

PM / devfreq: rockchip: add PM_DEVFREQ_EVENT dependency

The newly added ARM_RK3399_DMC_DEVFREQ driver requires the
DEVFREQ_EVENT_ROCKCHIP_DFI driver and tries to turn that on through
a 'select' statement, and that in turn has a dependency on
PM_DEVFREQ_EVENT, which may be disabled here:

warning: (ARM_RK3399_DMC_DEVFREQ) selects DEVFREQ_EVENT_ROCKCHIP_DFI which has unmet direct dependencies (PM_DEVFREQ && PM_DEVFREQ_EVENT && ARCH_ROCKCHIP)

We probably want a 'depends on' here, but other drivers use 'select'
too, so for consistency I'm doing the same.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 5a893e31a636 (PM / devfreq: rockchip: add devfreq driver for rk3399 dmc)
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>