tests: benchmark: Configure External timer to 32 bit mode for nrf SOCs

nrf SOCs use external timer for measuring benchmarking stats.
Earlier this timer is programmed in 24 bit mode, due to which
is is getting expired in some of benchmark stats like mutex
lock/unlock. So configuring to 32 bit mode to avoid overflow.

Signed-off-by: Youvedeep Singh <youvedeep.singh@intel.com>