soc: arm: rename soc/arm/nuvoton as soc/arm/nuvoton_numicro

We try to reduce the ambiguity between 'soc/nuvoton/' and
'soc/nuvoton_npcx' folders. Most if soc vendors name their soc folder by
the company name with SOC_FAMILY suffix instead of the company name
directly. It is clearer if the soc company has different product lines
that aim to specific markets.

In this CL, the 'numicro' suffix is used for Nuvoton Microcontroller
production line. It distinguishes the 'npcx' suffix that used for
Nuvoton Embedded Controller (EC) of Notebook PC product line.

Signed-off-by: Mulin Chao <mlchao@nuvoton.com>