ARM: shmobile: Remove legacy board code for KZM-A9-GT

The KZM-A9-GT board is sufficiently supported by DT-based and board-less
SH-Mobile AG5 (sh73a0) multiplatform kernels. Hence remove the legacy
board code to reduce maintenance effort.

Lacking areas are:
  - USB (it does't work in legacy, neither),
  - LCDC (the LCDC is wired to the legacy INTC, which is not planned to
    be supported with DT).
  - DMAC/IPMMU (no DT bindings are planned).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>