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/Makefile b/drivers/ata/Makefile
index e6e41b2..1fbc2aa 100644
--- a/drivers/ata/Makefile
+++ b/drivers/ata/Makefile
@@ -78,6 +78,7 @@
 # Should be last libata driver
 obj-$(CONFIG_PATA_LEGACY)	+= pata_legacy.o
 
-libata-objs	:= libata-core.o libata-scsi.o libata-eh.o libata-pmp.o
+libata-objs	:= libata-core.o libata-scsi.o libata-eh.o
 libata-$(CONFIG_ATA_SFF)	+= libata-sff.o
+libata-$(CONFIG_SATA_PMP)	+= libata-pmp.o
 libata-$(CONFIG_ATA_ACPI)	+= libata-acpi.o