powerpc/fsl_pci: Make fsl_add_bridge() static

Since commit 905e75c4 ("powerpc/fsl-pci: Unify pci/pcie initialization code")
fsl_add_bridge() is not used anymore outside of fsl_pci.c

Make it static.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/2115e3597d81e72a865820af54f0e290d0fd2b3a.1692199186.git.christophe.leroy@csgroup.eu