[nrf528xx] add OT_UNUSED_VARIABLE to avoid compiler warning (#4951)

When the macro `SPIS_AS_SERIAL_TRANSPORT` is enabled,
`UART_AS_SERIAL_TRANSPORT` and `USB_CDC_AS_SERIAL_TRANSPORT`
are disabled, no one uses the argument `aPseudoReset`. The
compiler generates a warning.

examples/platforms/nrf528xx/src/transport/transport.c

@@ -32,6 +32,8 @@
  *
  */
 
+#include <openthread/platform/toolchain.h>
+
 #include "platform-nrf5-transport.h"
 
 #include "transport-drivers.h"
@@ -50,6 +52,7 @@ void nrf5TransportInit(bool aPseudoReset)
 #endif
 
 #if (SPIS_AS_SERIAL_TRANSPORT == 1)
+    OT_UNUSED_VARIABLE(aPseudoReset);
     nrf5SpiSlaveInit();
 #endif
 }
@@ -64,6 +67,7 @@ void nrf5TransportDeinit(bool aPseudoReset)
 #endif
 
 #if (SPIS_AS_SERIAL_TRANSPORT == 1)
+    OT_UNUSED_VARIABLE(aPseudoReset);
     nrf5SpiSlaveDeinit();
 #endif
 }