arm: nordic_nrf5: Cleanup Kconfig dependencies for timer and clocks

On the nRF5x platforms we need always need the NRF_RTC_TIMER and it
depends on the CLOCK_CONTROL_NRF5.  So enable all of these always.

Fixes issues if one tries to build nRF5x platforms w/o CONFIG_BLUETOOTH.

Change-Id: I0f9af785e785f37ec289a935ddf70ee6dec08cd4
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>