ARM: shmobile: sh73a0: Introduce generic setup callback

Add a generic sh73a0 machine setup callback for the upcoming
sh73a0 multiplatform case. Cache needs to be configured, and
legacy clocks must be omitted.

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