dts: ti: cc13xx_cc26xx: devicetree sysclk alignment

This change introduces the "_rtc_timer" suffix for the system tick timer
driver "compatible" property and aligns naming conventions with the
actual CC13/26xx SoC series product policy.

This frees up the "_rtc" namespace to introduce additional APIs based on
the same peripheral in the future (not part of this PR):

rtc: rtc@... {
  compatible = "ti,cc13xx-cc26xx-rtc";
  ...

  timer {
    compatible = "ti,cc13xx-cc26xx-rtc-timer";
    ...
  };

  counter {
    compatible = "ti,cc13xx-cc26xx-rtc-counter";
    ...
  };

  pps {
    compatible = "ti,cc13xx-cc26xx-rtc-pps";
    ...
  };
};

Or alternatively an MFD pattern with similar requirements.

Fixing the namespacing now makes sense standalone as it reduces the
chance of custom drivers being broken in the future.

Redundant extension of the mandatory system clock devicetree node is
replaced with a single `status = "okay"` which seems to be the more
sensible default to avoid user error when defining custom boards.
Knowledgeable users can still override this if really needed.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>