zephyr/sample.yaml Limit allowed build platforms

Building sample.bootloader.mcuboot for many platforms
is not possible (for instance a qemu). The limit is need
as otherwise zephyr-rtos/zephyr CI is failing on any push to
main branch or nightly CI run.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/sample.yaml b/boot/zephyr/sample.yaml
index d8abab5..44ea89a 100644
--- a/boot/zephyr/sample.yaml
+++ b/boot/zephyr/sample.yaml
@@ -5,6 +5,7 @@
 tests:
   sample.bootloader.mcuboot:
     tags: bootloader_mcuboot
+    platform_allow:  nrf52840dk_nrf52840 frdm_k64f disco_l475_iot1
     integration_platforms:
       - nrf52840dk_nrf52840
       - frdm_k64f