When the switcher is active, there is no straightforward way to
figure out which logical CPU a given physical CPU maps to.

This patch provides a function
bL_switcher_get_logical_index(mpidr), which is analogous to
get_logical_index().

This function returns the logical CPU on which the specified
physical CPU is grouped (or -EINVAL if unknown).
If the switcher is inactive or not present, -EUNATCH is returned instead.
Signed-off-by: NDave Martin <dave.martin@linaro.org>
Signed-off-by: NNicolas Pitre <nico@linaro.org>