base-hw: Make sure MMU is initially disabled on ARMv8.

Genode code already expects MMU to be disabled when starting the
kernel. It is enabled eventually in Bootstrap::Platform::enable_mmu,
after setting up translation tables. Unfortunately nothing ensures
this is actually the case. If MMU happens to be enabled when entering
the kernel things go downhill pretty fast after we start messing with
TTBR.

This patch ensures MMU is disabled for EL1, EL2, EL3 dependent on the
exception level of the CPU core, which is entering the kernel.

This should allow base-hw to start correctly on Quartz64 A board.

repos/base-hw/src/bootstrap/spec/arm_64/crt0.s

@@ -28,6 +28,8 @@
 	.global _start
 	_start:
 
+	bl _mmu_disable
+
 	/**
 	 * Hack for Qemu, which starts all cpus at once
 	 * only first CPU runs through, all others wait for wakeup
@@ -45,6 +47,36 @@
 	.long 0
 
 
+	/*************************************
+	 ** Disable MMU of current EL (1-3) **
+	 *************************************/
+
+	_mmu_disable:
+	mrs  x8, CurrentEL
+	lsr  x8, x8, #2
+	cmp  x8, #0x2
+	b.eq _el2
+	b.hi _el3
+	_el1:
+	mrs x8, sctlr_el1
+	bic x8, x8, #(1 << 0)
+	msr sctlr_el1, x8
+	isb
+	ret
+	_el2:
+	mrs x8, sctlr_el2
+	bic x8, x8, #(1 << 0)
+	msr sctlr_el2, x8
+	isb
+	ret
+	_el3:
+	mrs x8, sctlr_el3
+	bic x8, x8, #(1 << 0)
+	msr sctlr_el3, x8
+	isb
+	ret
+
+
 	/***************************
 	 ** Zero-fill BSS segment **
 	 ***************************/
@@ -58,7 +90,6 @@
 	str xzr, [x1], #8
 	b 1b
 
-
 	/************************************
 	 ** Common Entrypoint for all CPUs **
 	 ************************************/
@@ -66,6 +97,8 @@
 	.global _crt0_start_secondary
 	_crt0_start_secondary:
 
+	bl _mmu_disable
+
 
 	/****************
 	 ** Enable FPU **