powerpc/85xx: mpc85xx_rdb: Do not automatically select FSL_ULI1575

Boards provided by CONFIG_MPC85xx_RDB option do not initialize
fsl_uli1575.c driver. So remove explicit select dependency on it.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230409000812.18904-6-pali@kernel.org