Implement Deref and DerefMut for Rcc

Cordoning off access to RCC registers makes implementing certain
software and drivers (e.g. a bootloader for STMF0 cores need SYSCFG)
unnecessarily difficult.

A completely encapsulated RCC renders peripheral enable registers
inaccessible and some drivers (e.g stm32_usbd) opt for using
unsafe raw pointer accesses completely eliminating the safety
and the structure provided by the HAL.

This commit copies an escape hatch from stm32f4xx-hal crate by
implementing Deref and DerefMut for Rcc structure.
The stm32f4xx-hal implementation can be seen at the link below:
https://github.com/stm32-rs/stm32f4xx-hal/blob/0ba01f4fb3f19b0eea40d8ea2403e2e9d4e41609/src/rcc/mod.rs#L56-L67

Author: ongun-kanat-egym

src/rcc.rs

@@ -1,5 +1,6 @@
 use crate::pac::RCC;
 use crate::time::Hertz;
+use core::ops::{Deref, DerefMut};
 
 /// Extension trait that sets up the `RCC` peripheral
 pub trait RccExt {
@@ -45,6 +46,21 @@ pub struct Rcc {
     pub(crate) regs: RCC,
 }
 
+impl Deref for Rcc {
+    type Target = RCC;
+
+    fn deref(&self) -> &Self::Target {
+        &self.regs
+    }
+}
+
+impl DerefMut for Rcc {
+
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.regs
+    }
+}
+
 pub enum HSEBypassMode {
     /// Not bypassed: for crystals
     NotBypassed,