ARM: lpc32xx: add missing include

lpc32xx_loopback_set() is defined in linux/soc/nxp/lpc32xx-misc.h but
this is not included before the function definition.

arch/arm/mach-lpc32xx/serial.c:63:6: error: no previous prototype for 'lpc32xx_loopback_set'

Fixes: ffba29c9 ("serial: lpc32xx: allow compile testing")
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Link: https://lore.kernel.org/r/20230516153109.514251-7-arnd@kernel.orgSigned-off-by: Arnd Bergmann <arnd@arndb.de>

arch/arm/mach-lpc32xx/serial.c

@@ -15,6 +15,7 @@
 #include <linux/serial_8250.h>
 #include <linux/clk.h>
 #include <linux/io.h>
+#include <linux/soc/nxp/lpc32xx-misc.h>
 
 #include "lpc32xx.h"
 #include "common.h"