metag: perf: fix build on Meta1

Meta1 doesn't support PERF_ICORE or PERF_CHAN registers resulting in
build errors due to missing definitions. Fix this with an ifdef matching
the one in asm/metag_mem.h.

The build errors (found by a randconfig):

arch/metag/kernel/perf/perf_event.c: In function 'metag_pmu_enable_counter':
arch/metag/kernel/perf/perf_event.c:639: error: 'PERF_ICORE0' undeclared (first use in this function)
arch/metag/kernel/perf/perf_event.c:639: error: (Each undeclared identifier is reported only once
arch/metag/kernel/perf/perf_event.c:639: error: for each function it appears in.)
arch/metag/kernel/perf/perf_event.c:643: error: 'PERF_CHAN0' undeclared (first use in this function)

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-metag@vger.kernel.org