Add `vmacc` back to the ISA doc

It is actually supported in the HW

Change-Id: I220dd3f9bda1f62cd7bb9201d1d44cbc09801458

docs/kelvin_isa.md

diff --git a/docs/kelvin_isa.md b/docs/kelvin_isa.md
index 97b8cdc..917aeec 100644
--- a/docs/kelvin_isa.md
+++ b/docs/kelvin_isa.md
@@ -197,6 +197,7 @@
 vmulw           | 04 xxxxxU |
 vmulh.{ur}      | 08 xxxxRU |
 vdmulh.{rn}     | 16 xxxxRN |
+vmacc           | 20 xxxxxx |
 vmadd           | 21 xxxxxx |
 **Float**       | ...       | **101**
 --reserved--    | xx xxxxxx |
@@ -1297,6 +1298,24 @@
 
 --------------------------------------------------------------------------------
 
+### VMACC
+
+Multiply accumulate.
+
+**Encodings**
+
+vmacc.[b,h,w].vv.{m} vd, vs1, vs2 \
+vmacc.[b,h,w].vx.{m} vd, vs1, xs2
+
+**Operation**
+
+```
+for L in Op.typelen
+  vd[N] += vs1[L] * vs2[L]
+```
+
+--------------------------------------------------------------------------------
+
 ### VMADD
 
 Multiply add.