stm32l4: flash: don't use misleading names

flash_clear_pgperr_flag is a name used on f247, which is actually most
analogous to the SIZERR bit on l4, (it's a parallelism error)

the bit being cleared originally in this function, PROGERR is a new bit,
and should have it's own name.

Add a function to handle the previously unhandled size/parallelism flag,
and rename the existing one to properly reflect it's new name.

include/libopencm3/stm32/l4/flash.h

@@ -222,8 +222,9 @@
 
 BEGIN_DECLS
 
-void flash_clear_pgperr_flag(void);
+void flash_clear_progerr_flag(void);
 void flash_clear_pgserr_flag(void);
+void flash_clear_size_flag(void);
 void flash_clear_pgaerr_flag(void);
 void flash_clear_wrperr_flag(void);
 void flash_lock_option_bytes(void);