Add ubnt-erx 5.15 support

root/target/linux/ramips/patches-5.15/324-mt7621-perfctr-fix.patch

@@ -0,0 +1,15 @@
+--- a/arch/mips/ralink/irq-gic.c
++++ b/arch/mips/ralink/irq-gic.c
+@@ -13,6 +13,12 @@
+ 
+ int get_c0_perfcount_int(void)
+ {
++	/*
++	 * Performance counter events are routed through GIC.
++	 * Prevent them from firing on CPU IRQ7 as well
++	 */
++	clear_c0_status(IE_SW0 << 7);
++
+ 	return gic_get_c0_perfcount_int();
+ }
+ EXPORT_SYMBOL_GPL(get_c0_perfcount_int);