Scala compile warning: Use foldLeft instead of /:

[ingallsj]

Type of change: other enhancement (paying off technical debt)
Impact: no functional change
Development Phase: implementation

fix these Chisel compile warnings:

[warn] /rocket-chip/src/main/scala/rocket/PMP.scala:139:38: method /: in trait TraversableOnce is deprecated (since 2.12.10): Use foldLeft instead of /:
[warn]     ((true.B, 0.U.asTypeOf(new PMP)) /: pmps) { case ((h, prev), pmp) =>
[warn]                                      ^
[warn] /rocket-chip/src/main/scala/rocket/PMP.scala:163:19: method /: in trait TraversableOnce is deprecated (since 2.12.10): Use foldLeft instead of /:
[warn]   val res = (pmp0 /: (io.pmp zip (pmp0 +: io.pmp)).reverse) { case (prev, (pmp, prevPMP)) =>
[warn]                   ^
[warn] /rocket-chip/src/main/scala/util/package.scala:32:17: method /: in trait TraversableOnce is deprecated (since 2.12.10): Use foldLeft instead of /:
[warn]         (x.head /: x.zipWithIndex.tail) { case (prev, (cur, i)) => Mux(truncIdx === i.U, cur, prev) }
[warn]                 ^
[warn] /rocket-chip/src/main/scala/util/package.scala:43:10: method /: in trait TraversableOnce is deprecated (since 2.12.10): Use foldLeft instead of /:
[warn]       (x /: (0 until log2Ceil(x.size)))((r, i) => (r.rotate(1 << i) zip r).map { case (s, a) => Mux(amt(i), s, a) })
[warn]          ^
[warn] /rocket-chip/src/main/scala/util/package.scala:51:10: method /: in trait TraversableOnce is deprecated (since 2.12.10): Use foldLeft instead of /:
[warn]       (x /: (0 until log2Ceil(x.size)))((r, i) => (r.rotateRight(1 << i) zip r).map { case (s, a) => Mux(amt(i), s, a) })
[warn]          ^
[warn] /rocket-chip/src/main/scala/util/package.scala:137:10: method /: in trait TraversableOnce is deprecated (since 2.12.10): Use foldLeft instead of /:
[warn]       (x /: (0 until log2Ceil(x.getWidth)))((r, i) => Mux(amt(i), r.rotateRight(1 << i), r))
[warn]          ^
[warn] /rocket-chip/src/main/scala/util/package.scala:144:10: method /: in trait TraversableOnce is deprecated (since 2.12.10): Use foldLeft instead of /:
[warn]       (x /: (0 until log2Ceil(x.getWidth)))((r, i) => Mux(amt(i), r.rotateLeft(1 << i), r))
[warn]          ^

[jackkoenig]

Not that it matters but this is a deprecation from the Scala collections library, not Chisel 😜