soc/sifive: fix compiler warning

Fix the following compiler warning on the latest toolchain:
src/soc/sifive/fu540/otp.c:48:1: error: useless storage class specifier in empty declaration [-Werror]
 } __packed;
 ^

Change-Id: Ice87c821de7650ac547394efa2a4bcc5ae1ea668
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/28553
Tested-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/soc/sifive/fu540/otp.c

@@ -29,7 +29,7 @@
  * https://www.sifive.com/documentation/chips/freedom-u540-c000-manual/
  */
 
-typedef struct sifive_otp_registers {
+struct sifive_otp_registers {
 	u32 pa;     /* Address input */
 	u32 paio;   /* Program address input */
 	u32 pas;    /* Program redundancy cell selection input */