Fix boot warning 'Trying to vfree() nonexistent vm area'
from arch_timer_mem_of_init().

Refactored code attempts to read and iounmap using address frame
instead of address ioremap(frame->cntbase).

Fixes: c389d701 ("clocksource: arm_arch_timer: split MMIO timer probing.")
Signed-off-by: NFrank Rowand <frank.rowand@sony.com>
Reviewed-by: NFu Wei <fu.wei@linaro.org>
Acked-by: NMarc Zyngier <marc.zyngier@arm.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>