ARM: at91: armv7: sama7g5 uses CCF clock driver

SAMA7G5 uses CCF driver under drivers/clk/at91/ and not the custom older
at91 clock.c driver. Remove it from the compilation list and adapt cpu.c
arch_cpu_init() to avoid calling at91_clock_init() which is wrong
anyway.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>