hw: enable performance counter for ARMv8

Ref #3426

repos/base-hw/src/bootstrap/spec/rpi3/platform.cc

@@ -104,6 +104,9 @@ unsigned Bootstrap::Platform::enable_mmu()
 			prepare_hypervisor();
 	}
 
+	/* enable performance counter for user-land */
+	Cpu::Pmuserenr_el0::write(0b1111);
+
 	Cpu::Vbar_el1::write(Hw::Mm::supervisor_exception_vector().base);
 
 	/* set memory attributes in indirection register */

repos/base-hw/src/include/hw/spec/arm_64/cpu.h

@@ -97,6 +97,8 @@ struct Hw::Arm_64_cpu
 		struct Attr3 : Bitfield<24, 8> {};
 	);
 
+	SYSTEM_REGISTER(32, Pmuserenr_el0, pmuserenr_el0);
+
 	SYSTEM_REGISTER(64, Scr, scr_el3,
 		struct Ns  : Bitfield<0,  1> {};
 		struct Smd : Bitfield<7,  1> {};