libata: make PMP support optional

Make PMP support optional by adding CONFIG_SATA_PMP and leaving out
libata-pmp.c if it isn't set.  PMP helpers return constant values if
PMP support is not enabled and PMP declarations alias non-PMP
counterparts.  This makes the compiler to leave out PMP related part
out and LLDs to use non-PMP counterparts automatically.

Signed-off-by: Tejun Heo <htejun@gmail.com>
diff --git a/drivers/ata/Kconfig b/drivers/ata/Kconfig
index 48c8fc5..3eb6035 100644
--- a/drivers/ata/Kconfig
+++ b/drivers/ata/Kconfig
@@ -41,6 +41,12 @@
 	  You can disable this at kernel boot time by using the
 	  option libata.noacpi=1
 
+config SATA_PMP
+	bool "SATA Port Multiplier support"
+	default y
+	help
+	  This option adds support for SATA Port Multipliers.
+
 config SATA_AHCI
 	tristate "AHCI SATA support"
 	depends on PCI