ARM: at91: Drop unneeded select of COMMON_CLK

Support for AT91/Microchip SoCs depends on ARCH_MULTIPLATFORM or
ARM_SINGLE_ARMV7M, which both select COMMON_CLK.
Hence there is no need for COMMON_CLK_AT91 to select COMMON_CLK.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/20200121103722.1781-5-geert+renesas@glider.be


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>