Basic helper routines for CRC

Note, the CRC block is pretty useless for interoperability.  It only operates on 32bit
chunks, and in a different bit order.  No attempt to make full helpers for compatibility
with other implementations has been done.

https://my.st.com/public/STe2ecommunities/mcu/Lists/cortex_mx_stm32/Flat.aspx?RootFolder=%2Fpublic%2FSTe2ecommunities%2Fmcu%2FLists%2Fcortex_mx_stm32%2FCRC%20computation&FolderCTID=0x01200200770978C69A1141439FE559EB459D7580009C4E14902C3CDE46A77F0FFD06506F5B&currentviews=2006

include/libopencm3/stm32/crc.h

@@ -55,4 +55,24 @@
 
 /* TODO */
 
+/**
+ * Reset the CRC calculator to initial values.
+ */
+void crc_reset(void);
+
+/**
+ * Add a word to the crc calculator and return the result.
+ * @param data new word to add to the crc calculator
+ * @return final crc calculator value
+ */
+u32 crc_calculate(u32 data);
+
+/**
+ * Add a block of data to the CRC calculator and return the final result
+ * @param datap pointer to the start of a block of 32bit data words
+ * @param size length of data, in 32bit increments
+ * @return final CRC calculator value
+ */
+u32 crc_calculate_block(u32 *datap, int size);
+
 #endif