clk: at91: clk-utmi: add support for sama7g5

Add UTMI support for SAMA7G5. SAMA7G5's UTMI control is done via
XTALF register. Values written at bits 2..0 in this register
correspond to the on board crystal oscillator frequency.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>