board/spear/common: move code to arch/arm/cpu/arm926ejs/spear/

board/spear/common: move code to arch/arm/cpu/arm926ejs/spear/

The code in board/spear/common/ is not board-specific but
SoC-specific. Therefore, boards from other vendors than "spear" may
want to re-use this code, which is currently difficult with the code
being placed in board/spear/common/.

Since this code really is SoC-specific, this commit moves it to
arch/arm/cpu/arm926ejs/spear/, with the rest of the SPEAr related
code.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>