add pmc, efc modules from @jamesmunns's work

Sourced from [0]; /vendor/asamx7x-hal.

[0] jamesmunns/same70-experiments@a6d1fe1

Author: jamesmunns

hal/src/efc.rs

@@ -0,0 +1,68 @@
+//! Flash block configuration
+
+use crate::target_device::EFC;
+use crate::pmc::PmcError;
+
+pub struct Efc {
+    pub(crate) periph: EFC,
+}
+
+impl Efc {
+    pub fn new(periph: EFC) -> Self {
+        periph.eefc_wpmr.modify(|_r, w| {
+            w.wpkey().passwd();
+            w.wpen().clear_bit();
+            w
+        });
+
+        Self { periph }
+    }
+
+    pub fn set_wait_states(&mut self, fws: FlashWaitStates) {
+        let fws_bits = fws as u8;
+
+        self.periph
+            .eefc_fmr
+            .modify(|_r, w| unsafe { w.fws().bits(fws_bits) });
+    }
+}
+
+/// The number of flash wait states for a read operation.
+///
+/// Note: The number of cycles a read takes is 1 + FWS.
+#[derive(Debug, PartialEq, Copy, Clone)]
+#[repr(u8)]
+pub enum FlashWaitStates {
+    Zero,
+    One,
+    Two,
+    Three,
+    Four,
+    Five,
+    Six,
+}
+
+impl FlashWaitStates {
+    /// Calculate the lowest possible number of flash wait states from a given
+    /// peripheral clock frequency in MHz.
+    ///
+    /// The max PCLK frequency supported is 150MHz. This is *not* the CPU frequency,
+    /// which may go up to 300MHz.
+    ///
+    /// Note: This is probably only valid at VDDIO = 3.0V
+    pub fn from_pclk_mhz(freq: u8) -> Result<Self, PmcError> {
+        // Reference: Table 58-51 Embedded Flash Wait States for Worst-Case Conditions
+        let fws = match freq {
+            0..=23 => Self::Zero,
+            24..=46 => Self::One,
+            47..=69 => Self::Two,
+            70..=92 => Self::Three,
+            93..=115 => Self::Four,
+            116..=138 => Self::Five,
+            139..=150 => Self::Six,
+            _ => return Err(PmcError::InvalidConfiguration),
+        };
+
+        Ok(fws)
+    }
+}

hal/src/lib.rs

@@ -145,3 +145,7 @@ pub use atsamv71q21b as target_device;
 pub mod serial;
 #[cfg(feature = "rev-b")]
 pub mod watchdog;
+#[cfg(feature = "rev-b")]
+pub mod pmc;
+#[cfg(feature = "rev-b")]
+pub mod efc;