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

arch/powerpc/platforms/Kconfig

@@ -261,7 +261,9 @@ config CPM2
 	  on it (826x, 827x, 8560).
 
 config FSL_ULI1575
-	bool
+	bool "ULI1575 PCIe south bridge support"
+	depends on FSL_SOC_BOOKE || PPC_86xx
+	select FSL_PCI
 	select GENERIC_ISA_DMA
 	help
 	  Supports for the ULI1575 PCIe south bridge that exists on some