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

Add master clock (MCK1..MCK4) support for SAMA7G5. SAMA7G5's PMC has
multiple master clocks feeding different subsystems. One of them
feeds image subsystem and is changeable based on image subsystem
needs.

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