soc: it8xxx2: The "M" extension is disabled by default

There is a mul instruction bug.
The bug may cause instructions of writing back CPU GPR (e.g mv a0,s2)
which following the mul instruction to fail.
This patch disables the 'M' extension and overwrite integer
multiplication and division arithmetic library routines with using
hardware multiplication and division and nop instructions.
This will ensure that there is no write back GPR instruction to follow
mul instruction to avoid the bug.

Signed-off-by: Dino Li <Dino.Li@ite.com.tw>