drivers: serial: fix zephyr.h warning

When compiling the cadence uart, a warning is generated for the
zephyr/zephyr.h include. Fix it to be zephyr/kernel.h.

Signed-off-by: Ryan McClelland <[email protected]>

Author: XenuIsWatching

drivers/serial/uart_cdns.h

@@ -8,7 +8,7 @@
 
 #include <zephyr/arch/cpu.h>
 #include <zephyr/drivers/uart.h>
-#include <zephyr/zephyr.h>
+#include <zephyr/kernel.h>
 
 enum csr_parity_val {
 	EVEN_PARITY_VAL,