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>