ARM: shmobile: sh73a0 dtsi: Add selectable sources to DIV6 clocks

Specifies clock sources and register bits.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
[geert: Drop renesas,src-shift/renesas,src-width, pad to 4 or 8 parents]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>