ARM: integrator: fix build with INTEGRATOR_AP off

The conditional declaration of ap_uart_data is broken
and causes this build error:

In file included from arch/arm/mach-integrator/core.c:35:0:
arch/arm/mach-integrator/common.h:6:37: error: expected '=', ',', ';', 'asm' or '__attribute__' before '{' token

Turning the check into an constant-expression if(IS_ENABLED()) statement
creates more readable code and solves this problem as well.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Russell King <linux@arm.linux.org.uk>

arch/arm/mach-integrator/common.h

 #include <linux/amba/serial.h>
-#ifdef CONFIG_ARCH_INTEGRATOR_AP
 extern struct amba_pl010_data ap_uart_data;
-#else
-/* Not used without Integrator/AP support anyway */
-struct amba_pl010_data ap_uart_data {};
-#endif
 void integrator_init_early(void);
 int integrator_init(bool is_cp);
 void integrator_reserve(void);

arch/arm/mach-integrator/core.c

@@ -71,7 +71,7 @@ int __init integrator_init(bool is_cp)
 	 * hard-code them. The Integator/CP and forward have proper cell IDs.
 	 * Else we leave them undefined to the bus driver can autoprobe them.
 	 */
-	if (!is_cp) {
+	if (!is_cp && IS_ENABLED(CONFIG_ARCH_INTEGRATOR_AP)) {
 		rtc_device.periphid	= 0x00041030;
 		uart0_device.periphid	= 0x00041010;
 		uart1_device.periphid	= 0x00041010;