tests: uart: Add nrf9160_pca10090 to UART async test

Add nrf9160_pca10090 to UART async test.
Add myself as codeowner to uart_async_api tests.

Signed-off-by: Mieszko Mierunski <mieszko.mierunski@nordicsemi.no>
diff --git a/CODEOWNERS b/CODEOWNERS
index a331c46..ed55d2c 100644
--- a/CODEOWNERS
+++ b/CODEOWNERS
@@ -279,6 +279,7 @@
 /tests/drivers/can/                       @alexanderwachter
 /tests/drivers/hwinfo/                    @alexanderwachter
 /tests/drivers/spi/                       @tbursztyka
+/tests/drivers/uart/uart_async_api/       @Mierunski
 /tests/kernel/                            @andrewboie @andyross @nashif
 /tests/net/                               @jukkar @tbursztyka @pfalcon
 /tests/net/buf/                           @jukkar @jhedberg @tbursztyka @pfalcon