ARM: dts: imx6ull: keep IMX6UL_ prefix for signals on both i.MX6UL and i.MX6ULL

ARM: dts: imx6ull: keep IMX6UL_ prefix for signals on both i.MX6UL and i.MX6ULL

Signals available on both i.MX6UL and i.MX6ULL should have the same name
because it is the case of all others common signals, it avoids to make
mistakes (use the wrong ones) and it makes writing device tree files
less complicated. For example:

imx6ul-imx6ull-board.dtsi:
 ...
 pinctrl_uart5: uart5grp {
	fsl,pins = <
		MX6UL_PAD_UART5_TX_DATA__UART5_DCE_TX   0x1b0b1
	 	MX6UL_PAD_UART5_RX_DATA__UART5_DCE_RX   0x1b0b1
 	 >;
 };

imx6ul-board.dts:
 #include <imx6ul.dtsi>
 #include <imx6ul-imx6ull-board.dtsi>
 ...

imx6ull-board.dts:
 #include <imx6ull.dtsi>
 #include <imx6ul-imx6ull-board.dtsi>
 ...

Without this patch, the imx6ull-board.dtb will use
MX6UL_PAD_UART5_RX_DATA__UART5_DCE_RX instead of
MX6ULL_PAD_UART5_RX_DATA__UART5_DCE_RX and the uart5 will be
misconfigured.

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>