arm64: disable the EL2 MMU before dropping to EL1

An earlier stage may have set HCR_EL2.E2H, the clearing of which may
break address translation.  We don't need the EL2 MMU at this point, so
we can avoid re-enabling it for now and just drop to EL1 as usual.

Suggested by:	andrew
Reviewed by:	andrew
Differential Revision:	https://reviews.freebsd.org/D34644

sys/arm64/arm64/locore.S

@@ -247,6 +247,16 @@ LENTRY(drop_to_el1)
 	b.eq	1f
 	ret
 1:
+	/*
+	 * Disable the MMU. If the HCR_EL2.E2H field is set we will clear it
+	 * which may break address translation.
+	 */
+	dsb	sy
+	mrs	x2, sctlr_el2
+	bic	x2, x2, SCTLR_M
+	msr	sctlr_el2, x2
+	isb
+
 	/* Configure the Hypervisor */
 	ldr	x2, =(HCR_RW | HCR_APK | HCR_API)
 	msr	hcr_el2, x2