powerpc/chrp: Make hydra_init() static

Commit 407d418f ("powerpc/chrp: Move PHB discovery") moved the
sole call to hydra_init() to the source file where it is defined, so it
can be made static.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210223095345.2139416-1-geert@linux-m68k.org