Remove board support for the sh7372 based AP4EVB board

The sh7372 SoC support code is still kept around since it
is in use by the Mackerel board which is basically a more
recent board where the design is based on AP4EVB.
Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>