[QCOM][QCS6490] Enable RPMSG and MAILBOX in defconfig

The MAILBOX and RPMSG drivers are required for the co-processor
communication on the QCS6490 (more specfic the QCOM_GLINK sub-system
depends on them). For whatever reasen, they have been disabled.
Re-enable them since we get compile and run time errors otherwise.

arch/arm64/configs/seco_qcs6490_defconfig

@@ -895,3 +895,5 @@ CONFIG_SPI_MT65XX=y
 CONFIG_SPI_SPIDEV=y
 CONFIG_SPI_SLAVE=y
 
+CONFIG_RPMSG=y
+CONFIG_MAILBOX=y