armv8: Initialize CNTFRQ if at highest exception level

CNTFRQ_EL0 is only writable from the highest supported exception
level on the platform. For Armv8-A, this is typically EL3, but
technically EL2 and EL3 are optional so it may need to be
initialized at EL2 or EL1. For Armv8-R, the highest exception
level is always EL2.

This patch moves the initialization outside of the switch_el
block and uses a new macro branch_if_not_highest_el which
dynamically detects whether it is at the highest supported
exception level.

Linux's docs state that CNTFRQ_EL0 should be initialized by the
bootloader. If not set, the the U-Boot prompt countdown hangs.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>