soc: fsl: cpm1: tsa: Add missing spinlock comment

checkpatch.pl raises the following issue
  CHECK: spinlock_t definition without comment

Add the missing comment.
Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/20240808071132.149251-7-herve.codina@bootlin.comSigned-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

drivers/soc/fsl/qe/tsa.c

@@ -128,7 +128,7 @@ struct tsa {
 	void __iomem *si_regs;
 	void __iomem *si_ram;
 	resource_size_t si_ram_sz;
-	spinlock_t	lock;
+	spinlock_t	lock; /* Lock for read/modify/write sequence */
 	int tdms; /* TSA_TDMx ORed */
 	struct tsa_tdm tdm[2]; /* TDMa and TDMb */
 	struct tsa_serial {