ARM: kirkwood: move SOC sources to mach-kirkwood

Move
arch/arm/cpu/arm926ejs/kirkwood/* -> arch/arm/mach-kirkwood/*

Note:
 Perhaps, can we merge arch/arm/mach-kirkwood and
 arch/arm/mvebu-common into arch/arm/mach-mvebu, like Linux?

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Stefan Roese <sr@denx.de>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
Cc: Luka Perkov <luka.perkov@sartura.hr>