gra: Avoid compiling cpuidle code when not configured

No need to compile cpuidle.c and sleep.S if cpuidle isn't configured in the
kernel.

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>
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