ARM: footbridge: remove RTC_IRQ definition

Since commit bd8abc9a "ARM: mc146818rtc: remove unnecessary include of
mach/irqs.h", building footbridge_defconfig results in this warning:

In file included from include/linux/mc146818rtc.h:16:0,
                 from arch/arm/mach-footbridge/isa-rtc.c:21:
arch/arm/include/asm/mc146818rtc.h:10:0: warning: "RTC_IRQ" redefined [enabled by default]
arch/arm/mach-footbridge/include/mach/irqs.h:93:0: note: this is the location of the previous definition

The above commit was intentionally made to catch errors like this,
where code relies on the RTC_IRQ definition. The only driver using
it is the legacy PC-style drivers/char/rtc.c driver.
However, the ARM architecture has been using the RTC_LIB framework
since at least 2006, and that doesn't use it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: rtc-linux@googlegroups.com
Cc: Russell King <linux@arm.linux.org.uk>