ata: Migrate CONFIG_SATA_SIL to Kconfig

Use 'imply' here liberally to avoid the combinatorial explosion of
defconfig changes in the PowerPC boards.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/drivers/ata/Kconfig b/drivers/ata/Kconfig
index 803064a..617c80f 100644
--- a/drivers/ata/Kconfig
+++ b/drivers/ata/Kconfig
@@ -47,4 +47,10 @@
 	  Enable this driver to support Sata devices through
 	  Synopsys DWC AHCI module.
 
+config SATA_SIL
+	bool "Enable Silicon Image SIL3131 / SIL3132 / SIL3124 SATA driver support"
+	help
+	  Enable this driver to support the SIL3131, SIL3132 and SIL3124
+	  SATA controllers.
+
 endmenu