hardcoded pointer with extern struct with padding for mmio

[justinbalexander]

Suggested by @markfirmware.

Pending on whether volatile keyword is removed in ziglang/zig#4284

pub fn mmio(address: u32, comptime mmio_type: type) *volatile mmio_type {
    return @intToPtr(*volatile mmio_type, address);
}

pub const Wdt = mmio(0x40010500, extern struct {
  start: u32,
  padding0: [(0x40010504 - 0x40010004) / 4]u32,
  counter_reset_value: u32,
  reload_request_enable: u32,
  padding1: [(0x40010600 - 0x400105C0) / 4]u32,
  reload_request: [8]u32,
});