arch-riscv: Fix reset function and style

arch-riscv: Fix reset function and style

In addition to fixing some style issues with resetting, this patch fixes
what happens on reset. The RISC-V privileged ISA reference manual says
that,
on reset:
 1. Privilege mode is set to M
 2. mstatus.mie <- 0; mstatus.mprv <- 0
 3. PC <- reset vector
 4. mcause <- reset cause (0 if there is no distinguishing causes)
 5. Everything else is undefined
Because of 5, everything else will be left alone

Change-Id: I81bdf7a88b08874e3c3d5fc6c7f3ca2d796496b8
Reviewed-on: https://gem5-review.googlesource.com/c/14376
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>