armv8: Handle EL2 Host mode

On implementations that support VHE, the layout of the CPTR_EL2
register depends on whether HCR_EL2.E2H is set.  If the bit is
set, CPTR_EL2 uses the same layout as CPACR_EL1 and can in fact
be accessed through that register.  In that case, jump to the
EL1 code to enable access to the FP/SIMD registers.  This allows
U-Boot to run on systems that pass control to U-Boot in EL2 with
EL2 Host mode enabled such as machines using Apple's M1 SoC.
Signed-off-by: NMark Kettenis <kettenis@openbsd.org>
Acked-by: NMarc Zyngier <maz@kernel.org>

arch/arm/cpu/armv8/start.S

@@ -132,11 +132,13 @@ pie_fixup_done:
 	msr	cntfrq_el0, x0			/* Initialize CNTFRQ */
 #endif
 	b	0f
-2:	set_vbar	vbar_el2, x0
+2:	mrs	x1, hcr_el2
+	tbnz	x1, #34, 1f			/* HCR_EL2.E2H */
+	set_vbar vbar_el2, x0
 	mov	x0, #0x33ff
 	msr	cptr_el2, x0			/* Enable FP/SIMD */
 	b	0f
-1:	set_vbar	vbar_el1, x0
+1:	set_vbar vbar_el1, x0
 	mov	x0, #3 << 20
 	msr	cpacr_el1, x0			/* Enable FP/SIMD */
 0: