Add a map_addr method for transforming a register address

[jnkr-ifx]

On many processors implementing TrustZone, bit 28 of the address space is used to indicate whether a memory access is secure or non-secure, meaning drivers often need to bitwise-OR a security bit against the register address.

In order to support this with svd2pac, this commit adds a map_addr function to registers and clusters allowing a driver to override the register's address:

/// Creates a new register by mapping the addresss of this register. This is useful on platforms where
/// some bits of a transaction address are used to indicate flags (such as a TrustZone security attribute).
///
/// # Safety
/// The address must be non-null, as well as valid and properly aligned for the read and write operations
/// performed on the resulting register instance.
#[must_use]
unsafe fn map_addr(&self, f: impl FnOnce(usize) -> usize) -> &Self;

The function is implemented as a trait method for two reasons:

• To avoid naming collisions in case a cluster has a register named map_addr.
• So that drivers can build safe abstractions on top of map_addr, like:

/// Helper trait for applying a security attribute to a register address.
pub trait WithSecurity {
    /// Applies a security attribute to the address referenced by `self`.
    fn with_security(self, security: impl Security) -> Self;
}

impl<T: pac::MapAddr> WithSecurity for &T {
    fn with_security(self, security: impl Security) -> Self {
        unsafe { self.map_addr(|addr| security.map_addr(addr)) }
    }
}

The function signature intentionally mirrors std's map_addr function on pointer types.