imx:mx6ul add clock support

1. Add enet, uart, i2c, ipg clock support for i.MX6UL.
2. Correct get_periph_clk, it should account for
   MXC_CCM_CBCDR_PERIPH_CLK2_PODF_MASK.
3. Refactor get_mmdc_ch0_clk to make all i.MX6 share one function,
   but not use 'ifdef'.
4. Use CONFIG_FSL_QSPI for enable_qspi_clk, but not #ifdef CONFIG_MX6SX.
5. Use CONFIG_PCIE_IMX for pcie clock settings, use CONFIG_CMD_SATA for
   sata clock settings. In this way, we not need "#if defined(CONFIG_MX6Q)
   || defined....", only need one CONFIG_PCIE_IMX in header file.

Signed-off-by: Ye.Li <B37916@freescale.com>
Signed-off-by: Peng Fan <Peng.Fan@freescale.com>