xen/arm: don't read aarch32 regs when aarch32 isn't available

xen/arm: don't read aarch32 regs when aarch32 isn't available

Don't read aarch32 system registers at boot time when the aarch32 state
is not available at EL0. They are UNKNOWN, so it is not useful to read
them. Moreover, on Cavium ThunderX reading ID_PFR2_EL1 generates an
unsupported exception which causes a Xen crash.  Instead, only read them
when aarch32 is available.

Leave the corresponding fields in struct cpuinfo_arm so that they
are read-as-zero from a guest.

Since we are editing identify_cpu, also fix the indentation: 4 spaces
instead of 8.

Fixes: 9cfdb489af81 ("xen/arm: Add ID registers and complete cpuinfo")
Link: https://lore.kernel.org/xen-devel/f90e40ee-b042-6cc5-a08d-aef41a279527@suse.com/
Suggested-by: Julien Grall <julien@xen.org>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>