Document immediate sign-extend for PULP post-increment loads/stores

Created by: Silabs-ArjanB

For the following instructions the documentation does not indicate that the immediate is sign-extended:

• p.lb rD, Imm(rs1!)
• p.lbu rD, Imm(rs1!)
• p.lh rD, Imm(rs1!)
• p.lhu rD, Imm(rs1!)
• p.lw rD, Imm(rs1!)
• p.sb rs2, Imm(rs1!)
• p.sh rs2, Imm(rs1!)
• p.sw rs2, Imm(rs1!)

My proposal would be to replace 'rs1 += Imm[11:0]' by 'rs1 += Sext(Imm[11:0])' in their descriptions.