Unlike Linux, we don't require barebox drivers to support old device
tree bindings. Following device tree compatibles are no longer present
in barebox latest import of dts/:

 * atmel,sama5d2-clk-generated
 * atmel,sama5d4-clk-h32mx
 * atmel,at91sam9g45-clk-pll
 * atmel,at91sam9g20-clk-pllb
 * atmel,at91sam9260-clk-slow

Drop their compat code.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>