ARM: dts: imx51-babbage: Fix the 26MHz clock modelling

On imx51-babbage there is a 26MHz oscillator that is gated by GPIO3_1.

The output of this clock feeds audio codec clock and USB PHY clocks,
which are gated by GPIO4_26 and GPIO2_1 respectively.

Fix the clock representation by properly using gpio-gate-clock.

The clock nodes can be moved out of the 'clocks' node.

Based on a commit from Lucas Stach for imx51-zii-rdu1 board.

This also fixes the following build warning with W=1:

arch/arm/boot/dts/imx51-babbage.dtb: Warning (unit_address_vs_reg): Node /clocks/codec_clock has a reg or ranges property, but no unit name

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>