powerpc/fsl_uli1575: Allow to disable FSL_ULI1575 support

ULI1575 PCIe south bridge exists only on some Freescale boards. Allow to
disable CONFIG_FSL_ULI1575 symbol when it is not explicitly selected and
only implied. This is achieved by marking symbol as visible by providing
short description. Also adds dependency for this symbol to prevent enabling
it on platforms on which driver does not compile.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230409000812.18904-7-pali@kernel.org