Added SPI implementation

- First SPI HAL implementation for blocking mode
- Added example for SPI as well which uses loopback mode
  and regular mode

src/lib.rs

@@ -1,17 +1,17 @@
 #![no_std]
 
 pub use va108xx;
+pub use va108xx as pac;
 
 pub mod clock;
 pub mod gpio;
 pub mod prelude;
+pub mod spi;
 pub mod time;
 pub mod timer;
 pub mod typelevel;
 pub mod uart;
 
-pub use va108xx as pac;
-
 mod private {
     /// Super trait used to mark traits with an exhaustive set of
     /// implementations