arm: imx: imx8mq: add support to get values for more clocks

arm: imx: imx8mq: add support to get values for more clocks

Return the root clock values for MXC_CSPI_CLK, MXC_I2C_CLK,
MXC_UART_CLK and MXC_QSPI_CLK.

At least for the I2C clock the missing support leads to a wrong
configured I2C frequency. The expected value is 100kHz but the resulting
value is about 1MHz.

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>