fix(cpus): check for SME presence in Gelas

The original powerdown function for Gelas included SME disabling
instructions but did not check for the presence of SME before disabling.
This could lead to unexpected beaviors. This patch adds that check so
the feature is disabled only if it is present.

Change-Id: I582db53a6669317620e4f72a3eac87525897d3d0
Signed-off-by: Juan Pablo Conde <juanpablo.conde@arm.com>
diff --git a/include/arch/aarch64/arch.h b/include/arch/aarch64/arch.h
index bc37c94..951f023 100644
--- a/include/arch/aarch64/arch.h
+++ b/include/arch/aarch64/arch.h
@@ -437,6 +437,7 @@
 
 #define ID_AA64PFR1_EL1_SME_SHIFT		U(24)
 #define ID_AA64PFR1_EL1_SME_MASK		ULL(0xf)
+#define ID_AA64PFR1_EL1_SME_WIDTH		U(4)
 #define ID_AA64PFR1_EL1_SME_NOT_SUPPORTED	ULL(0x0)
 #define ID_AA64PFR1_EL1_SME_SUPPORTED		ULL(0x1)
 #define ID_AA64PFR1_EL1_SME2_SUPPORTED		ULL(0x2)