arch/riscv: add constants for Base Extension

Get used to this rate of change, SBI adds one new function a month,
on average, for the last 7 years.

Signed-off-by: Ronald G Minnich <rminnich@gmail.com>

Change-Id: Iaad763464678d1921dfefdbee1e39fba2fe5585a
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81286
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/arch/riscv/include/sbi.h

@@ -13,6 +13,19 @@
 #define SBI_REMOTE_SFENCE_VMA_ASID 7
 #define SBI_SHUTDOWN               8
 
+// Here begins the complex set of things added by
+// later SBI. Unlike the original design, these
+// require bits of state to be maintained in SBI.
+// Disappointing!
+#define SBI_BASE_EXTENSION 0x10
+#  define SBI_GET_SBI_SPEC_VERSION    0
+#  define SBI_GET_SBI_IMPL_ID         1
+#  define SBI_GET_SBI_IMPL_VERSION    2
+#  define SBI_PROBE_EXTENSION         3
+#  define SBI_GET_MVENDORID           4
+#  define SBI_GET_MARCHID             5
+#  define SBI_GET_MIMPID              6
+
 #define SBI_ENOSYS 38
 
 #define IPI_SOFT            1