arm: change tbu/l type to unsigned int

Change tbu/l type to unsigned int.
>From the timer file for arm,
"(((unsigned long long)gd->arch.tbu) << 32) | gd->arch.tbl;" is used,
This piece code is based on tbu/tbl is 32bits, so change the type to
unsigned int.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>