armv8: define get_ticks() for the ARMv8 Generic Timer

armv8: define get_ticks() for the ARMv8 Generic Timer

For 64-bit ARM systems we provide just a timer_read_counter()
implementation and rely on the generic non-uclass get_ticks() function
in lib/time.c to call the former.
However this function is actually not 64-bit safe, as it assumes a
"long" to be 32-bit. Beside the fact that the resulting uint64_t
isn't bigger than "long" on 64-bit architectures and thus combining two
counters makes no sense, we get all kind of weird results when we try
to OR in the high value shifted by _32_ bits.
So let's avoid that function at all and provide a straight forward
get_ticks() implementation for ARMv8, which also is in line with ARMv7.

This fixes occasional immediate time-out expiration issues I see on the
Pine64 board. The root cause of this needs to be investigated, but this
fix looks like the right thing anyway.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>