ARM: i.MX27 clk: dts: Use clock defines in DTS files

Use clock definitions in i.MX27 DTS files.
Additional changes included in this patch (imx27.dtsi):
- Fix IPG clock for UART6.
- Use EMI_AHB_GATE clock for WEIM.
- Added GPIO_IPG_GATE clock for GPIO nodes. Currently this clock is
  not used by the driver, but it can be added in the future.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>