Add udelay() support to generate a delay in the guest.

The routines are derived and simplified from kernel's
arch/arm64/lib/delay.c.
Signed-off-by: NRaghavendra Rao Ananta <rananta@google.com>
Reviewed-by: NAndrew Jones <drjones@redhat.com>
Reviewed-by: NOliver Upton <oupton@google.com>
Signed-off-by: NMarc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211007233439.1826892-8-rananta@google.com