arm: imx-common: introduce back usec2ticks

This commit "2bb01482"
do some clean up to use the code in lib/time.c.
But usec2ticks is still being used by security related job ring code.
Bring back the function to avoid build break when CONFIG_FSL_CAAM
is defined.
The computation logic has been changed, using 64-bit variable
to ease the process, making it work on older (MX5) platforms.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>