ARM: dts: r8a7791: Don't disable referenced optional clocks

clk_get on a disabled clock node will return EPROBE_DEFER, which can
cause drivers to be deferred forever if such clocks are referenced in
their clocks property.

Update the various disabled external clock nodes to default to a
frequency of 0, but don't disable them to prevent this.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>