ARM: shmobile: sh73a0: Remove unused sh73a0_add_standard_devices_dt()

"ARM: shmobile: kzm9g-reference: Remove board C code and DT file"
removed the last user of sh73a0_add_standard_devices_dt(). Hence remove
this function.

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