Remove unused files

[1] arch/arm/include/asm/arch-at91/at91_shdwn.h
 The top9000 was the last board to use this header file.
 It was removed by commit d58a9451 (ppc/arm: zap EMK boards).

[2] board/matrix_vision/common/*
 Some Matrix Vision boards were dropped by commit e7a56563
 (powerpc: mpc83xx: remove board support for MERGERBOX and MVBLM7)
 and commit af55e35d
 (powerpc: mpc5xxx: remove board support for MVBC_P and MVSMR).
 Since then these files have been unused.

[3] include/usb/omap1510_udc.h
  The omap5912osk was the last board to use this header file.
  It was removed by commit 62d636aa
  (omap: remove omap5912osk board support).

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-By: Wolfgang Denk <wd@denx.de>