ARM platforms: Add support for MT bit in MPIDR

This patch modifies some of the functions in ARM platform layer to cater
for the case when multi-threading `MT` is set in MPIDR. A new build flag
`ARM_PLAT_MT` is added, and when enabled, the functions accessing MPIDR
now assume that the `MT` bit is set for the platform and access the bit
fields accordingly.

Also, a new API plat_arm_get_cpu_pe_count is added when `ARM_PLAT_MT` is
enabled, returning the PE count within the physical cpu corresponding to
`mpidr`.

Change-Id: I04ccf212ac3054a60882761f4087bae299af13cb
Signed-off-by: Summer Qin <summer.qin@arm.com>
diff --git a/plat/arm/common/arm_common.mk b/plat/arm/common/arm_common.mk
index 4628a43..204ae4c 100644
--- a/plat/arm/common/arm_common.mk
+++ b/plat/arm/common/arm_common.mk
@@ -90,6 +90,11 @@
 $(eval $(call assert_boolean,ARM_BL31_IN_DRAM))
 $(eval $(call add_define,ARM_BL31_IN_DRAM))
 
+# Process ARM_PLAT_MT flag
+ARM_PLAT_MT			:=	0
+$(eval $(call assert_boolean,ARM_PLAT_MT))
+$(eval $(call add_define,ARM_PLAT_MT))
+
 # Enable PSCI_STAT_COUNT/RESIDENCY APIs on ARM platforms
 ENABLE_PSCI_STAT		:=	1
 ENABLE_PMF			:=	1