DO NOT MERGE: Fix incorrect TARGET_2ND_CPU_VARIANT for A/B devices

TARGET_2ND_CPU_VARIANT for non A/B devices has been changed to
cortex-a15 for better performance. Do the same for A/B devices.

Bug: 65042524
Test: build with aosp_arm64_ab
Change-Id: Id67c2779d2170f29b278c1c8a9ac513aa76a620d
diff --git a/target/board/generic_arm64_ab/BoardConfig.mk b/target/board/generic_arm64_ab/BoardConfig.mk
index 22b9f42..6744289 100644
--- a/target/board/generic_arm64_ab/BoardConfig.mk
+++ b/target/board/generic_arm64_ab/BoardConfig.mk
@@ -26,7 +26,7 @@
 TARGET_2ND_ARCH_VARIANT := armv7-a-neon
 TARGET_2ND_CPU_ABI := armeabi-v7a
 TARGET_2ND_CPU_ABI2 := armeabi
-TARGET_2ND_CPU_VARIANT := generic
+TARGET_2ND_CPU_VARIANT := cortex-a15
 
 # Enable A/B update
 TARGET_NO_RECOVERY := true