Fixup `SMCCC_ARCH_FEATURES` semantics

When querying `SMCCC_ARCH_WORKAROUND_1` through `SMCCC_ARCH_FEATURES`,
return either:
  * -1 to indicate the PE on which `SMCCC_ARCH_FEATURES` is called
    requires firmware mitigation for CVE-2017-5715 but the mitigation
    is not compiled in.
  * 0 to indicate that firmware mitigation is required, or
  * 1 to indicate that no firmware mitigation is required.

This patch complies with v1.2 of the firmware interfaces
specification (ARM DEN 0070A).

Change-Id: Ibc32d6620efdac6c340758ec502d95554a55f02a
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>
diff --git a/lib/cpus/aarch64/cortex_a75.S b/lib/cpus/aarch64/cortex_a75.S
index d102795..288f5af 100644
--- a/lib/cpus/aarch64/cortex_a75.S
+++ b/lib/cpus/aarch64/cortex_a75.S
@@ -113,6 +113,7 @@
 	ret
 endfunc cortex_a75_cpu_reg_dump
 
-declare_cpu_ops cortex_a75, CORTEX_A75_MIDR, \
+declare_cpu_ops_workaround_cve_2017_5715 cortex_a75, CORTEX_A75_MIDR, \
 	cortex_a75_reset_func, \
+	check_errata_cve_2017_5715, \
 	cortex_a75_core_pwr_dwn