sim: Implement optParamIn using paramIn.

sim: Implement optParamIn using paramIn.

This means only paramIn needs to be specialized, and then optParamIn
will be as well for free. It also removes some duplicate implementation.

Change-Id: Id124a05d04e1c0897121d0e13dd46efe90e8eed0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36276
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>