drivers: spi: nxp: flexiospi spi_loopback test failed on flexio spi

Several reason cause loopback test failed:
a) FlexIO input frequency is not correct, on RT11xx, input freq is 24M,
while max baud rate can reach 1/4 of input freq, so it can only support
6Mbps.
b) Flexio shift register depend on correct timer output to triggger TX
and RX, if timer comparison value is not accurate, RX error happens on
high baud rate. This is the reason why test fails on RT1060.

also fix a error on FlexIO clock ID calculation.

Signed-off-by: Raymond Lei <raymond.lei@nxp.com>