boards: arm64: xenvm: read real frequency

Read real frequncy from ARM Arch Timer instead of using define
'CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC' which can be incorrect for
some run cases. If we run xenvm under qemu we get one frequency, but
we can run this build as a DomU for Xen under different real platforms
and thus with differenty arch timer frequencies. So, we need to read
frequency from the timer registers.

Signed-off-by: Mykola Kvach <xakep.amatop@gmail.com>