imx-common: timer: fix 32-bit overflow

The i.MX6 common timer uses the 32-bit variable tbl (time base lower)
to record the overflow of the 32-bit counter. I.e. if the counter
overflows, the variable tbl does overflow, too.

To capture this overflow, use the variable tbu (time base upper), too.
Return the combined value of tbl and tbu.

lastinc is unused then, remove it.

Signed-off-by: Knut Wohlrab <knut.wohlrab@de.bosch.com>
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Stefano Babic <sbabic@denx.de>