MIPS: lantiq: Use CP0_LEGACY_COMPARE_IRQ

Instead of using the lantiq specific MIPS_CPU_TIMER_IRQ use the generic
CP0_LEGACY_COMPARE_IRQ constant for the timer interrupt number.
MIPS_CPU_TIMER_IRQ was already defined to 7 for both supported SoC
families.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: jhogan@kernel.org
Cc: ralf@linux-mips.org
Cc: john@phrozen.org
Cc: linux-mips@linux-mips.org
Cc: linux-mips@vger.kernel.org