ad7606x: Add dynamic configuration for AD7606X operation modes

[alin724]

axi_ad7606x_pi:

• add dynamic configuration for AD7606X operation modes using REG_CNTRL_3 reg;

AD7606x operation mode configuration:
REG_CNTRL_3
bit 8 - 'b1 - set operation mode indicated in bits [7:0];
bit [7:0] - set desired operation mode: 0 - SIMPLE, 1 - CRC, 2 - STATUS_HEADER, 3 - CRC_STATUS

[IuliaCMoldovan]

Also check the guideline action for errors

[alin724]

V2:

• solved code style issues;

[alin724]

V3:

• added commit description;

[alin724]

V4:

• updated ad7606x_fmc.txt file;
• delete empty lines from *.tcl files;

[alin724]

V5:

• Rebased to master branch;
• Renamed ADC_TO_DMA_N_BITS to ADC_CH_DW;
• Updated adc_config_mode logic;

[AndreiGrozav]

Update the wiki documentation for adc_custom_control at
https://wiki.analog.com/resources/fpga/docs/hdl/regmap_adc_custom

[alin724]

V6:

• Removed FIRST_DATA related gpio as it wouldn't be used int the initial version of the Linux driver (for IIO_CHAN_INFO_RAW functionality);