powerpc/85xx: mpc85xx_ds: Move uli_init() code into its own driver file

Move uli_init() function into existing driver fsl_uli1575.c file in order
to share its code between more platforms and board files.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230409000812.18904-5-pali@kernel.org