riscv: generate Pmpcsr via csr! macro

[JurajSadel]

Pmpcsr has a public bits: usize field, which is not consistent with others.

Breaking change

• Pmpcsr.bits (public field) removed → use .bits() or .into()
• Minor version bump expected

Migration:

// before
let raw = pmpcfg0::read().bits;
// after
let raw = pmpcfg0::read().bits();