fix(data): fix field names to be closer to operands

- `dret`, `mnret`, and `sctrclr` take no operands.
- `sspush` and `sspopchk` are each single mnemonics that take a very
  restricted set of operand values.
  Also include a bit of documentation.
- `vaeskf1.vi`, `vaeskf2.vi`, `vsm3c.vi`, `vsm4k.vi`, `vwsll.vi`:
  renamed field to match operand.

Author: ThinkOpenly

spec/std/isa/inst/Sdext/dret.yaml

@@ -10,7 +10,7 @@ long_name: No synopsis available
 description: |
   No description available.
 definedBy: Sdext
-assembly: dret
+assembly: ""
 encoding:
   match: "01111011001000000000000001110011"
   variables: []

spec/std/isa/inst/Smdbltrp/sctrclr.yaml

@@ -10,7 +10,7 @@ long_name: No synopsis available
 description: |
   No description available.
 definedBy: Smdbltrp
-assembly: sctrclr
+assembly: ""
 encoding:
   match: "00010000010000000000000001110011"
   variables: []

spec/std/isa/inst/Smrnmi/mnret.yaml

@@ -14,7 +14,7 @@ description: |
   also sets mstatus.MPRV to 0. If the Zicfilp extension is implemented, then if the new privileged mode is
   y, MNRET sets ELP to the logical AND of yLPE (see Section 22.1.1) and mnstatus.MNPELP.
 definedBy: Smrnmi
-assembly: mnret
+assembly: ""
 encoding:
   match: "01110000001000000000000001110011"
   variables: []

spec/std/isa/inst/Zicfiss/sspopchk.x1.yaml

@@ -18,7 +18,7 @@ encoding:
       location: 25-25
     - name: vs2
       location: 24-20
-    - name: zimm5
+    - name: imm
       location: 19-15
     - name: vd
       location: 11-7

spec/std/isa/inst/Zicfiss/sspopchk.x5.yaml

@@ -16,7 +16,7 @@ encoding:
   variables:
     - name: vs2
       location: 24-20
-    - name: zimm5
+    - name: imm
       location: 19-15
     - name: vd
       location: 11-7

spec/std/isa/inst/Zicfiss/sspush.x1.yaml

@@ -16,7 +16,7 @@ encoding:
   variables:
     - name: vs2
       location: 24-20
-    - name: zimm5
+    - name: imm
       location: 19-15
     - name: vd
       location: 11-7