ARM: shmobile: Remove legacy SoC code for SH-Mobile AG5

The last user of the SH-Mobile AG5 (sh73a0) legacy SoC code was the
KZM-A9-GT legacy board code, which has been removed.

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