ARM: at91: collect SoC sources into mach-at91

This commit moves source files as follows:

  arch/arm/cpu/arm920t/at91/*   -> arch/arm/mach-at91/arm920t/*
  arch/arm/cpu/arm926ejs/at91/* -> arch/arm/mach-at91/arm926ejs/*
  arch/arm/cpu/armv7/at91/*     -> arch/arm/mach-at91/armv7/*
  arch/arm/cpu/at91-common/*    -> arch/arm/mach-at91/*

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Andreas Bießmann <andreas.devel@googlemail.co>