[nrf fromtree] tests: drivers: Add PPR support in NRF GRTC timer test

Test did not support PPR core due to failing GRTC channel allocation.
This change fixes the channel allocation and enables PPR core test.

Signed-off-by: Bartosz Miller <bartosz.miller@nordicsemi.no>

(cherry picked from commit e26b9750ae32cf8232bf510a815c7b1eb93b4bfe)

tests/drivers/timer/nrf_grtc_timer/boards/nrf54h20dk_nrf54h20_cpuppr.overlay

@@ -0,0 +1,5 @@
+/* SPDX-License-Identifier: Apache-2.0 */
+
+&grtc {
+ owned-channels = <5 6>;
+};