clk: at91: sama7g5: register cpu clock

clk: at91: sama7g5: register cpu clock

Register CPU clock as being the master clock prescaler. This would
be used by DVFS. The block schema of SAMA7G5's PMC contains also a divider
between master clock prescaler and CPU (PMC_CPU_RATIO.RATIO) but the
frequencies supported by SAMA7G5 could be directly received from
CPUPLL + master clock prescaler and the extra divider would do no work in
case it would be enabled.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/1605800597-16720-12-git-send-email-claudiu.beznea@microchip.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>