aarch64: Document what we're doing when setting ZCR_EL3.LEN

The enumeration and configuration algorithm for SVE vector lengths is not
100% obvious so add a comment explaining what's going on in case someone
looks at this code as a reference. If this is ever used on hardware with
asymmetric vector lengths we will need to handle this differently to meet
Linux's boot requirements but this is not a present issue and such hardware
would be fairly surprising.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20220201172132.2399026-1-broonie@kernel.org
diff --git a/arch/aarch64/init.c b/arch/aarch64/init.c
index aa58567..8bb0524 100644
--- a/arch/aarch64/init.c
+++ b/arch/aarch64/init.c
@@ -88,6 +88,10 @@
 		cptr |= CPTR_EL3_EZ;
 		msr(CPTR_EL3, cptr);
+		/*
+		 * Write the maximum possible vector length, hardware
+		 * will constrain to the actual limit.
+		 */
 		msr(ZCR_EL3, ZCR_EL3_LEN_MAX);