ARM: at91/dt: at91sam9261: fix clocks and clock-names in udc definition

Peripheral clock is named pclk and system clock is named hclk (those are
the names expected by the at91_udc driver).

Drop the deprecated usb_clk (formerly used to configure the usb clock rate
which is now directly configurable through hclk).

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>