ARM: shmobile: mackerel: Remove Legacy C board code

Remove legacy C code for the sh7372 Mackerel board. There is no
DT multiplatform implementation available for the sh7372 SoC so
simply phase out the board and SoC code support.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>