aarch64: add HWCAP_SSBS from linux v4.20

for armv8.5 speculative store bypass PSTATE bit support,
added in linux commit d71be2b6c0e19180b5f80a6d42039cc074a693a2

arch/aarch64/bits/hwcap.h

@@ -26,3 +26,4 @@
 #define HWCAP_USCAT		(1 << 25)
 #define HWCAP_ILRCPC		(1 << 26)
 #define HWCAP_FLAGM		(1 << 27)
+#define HWCAP_SSBS		(1 << 28)