MAINTAINERS: Remove the AT91 clk driver entry

I've stopped actively maintaining this driver for quite some time
already, and at91 maintainers are doing a good job at maintaining it.
Remove the AT91 clk driver entry so that the driver automatically
falls under the "ARM/Microchip (AT91) SoC support" entry.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>