Smcsps: Clarify that stack pointer swapping always operates on x2

[christian-herber-nxp]

@pkennedyTT please review

[pkennedyTT]

Thanks for the clarification! This is the opposite of what I had assumed.

The only suggestions I would make are:

• Remove the "rd, rs" from the mnemonic description for mcspspush and scspspush (pp. 11, 13)
• Replace "rd" and "rs" with "sp" (or "x2") in the behavior sections for all four instructions (pp. 11 - 14)
• Remove the "sp, sp" operands from "mcspspop" in the behavior of "mipopret", likewise for "sipopret" (pp. 26 - 27)