Add four-channel support for GPIO source

[andsunds]

Issue Description

We intend to use the X440 as a digital back-end for various projects, and we would like to use the ATR_TX GPIO functionality to control the power amplifiers in the later Tx stages. However, it seems like it is only possible to set the GPIO source to either channel 0 or 1 on each radio daughter board. Trying to set, e.g., mbc.set_gpio_src("GPIO0", ["DB1_RF0","DB1_RF1","DB1_RF2","DB1_RF3"]) results in an error Invalid GPIO source name. For us it will be important to be able to control the amplifiers for each channel individually, so it would be good to add support for using all four channels available to the X440 as sources for the GPIO.

Setup Details

I'm using the X440 with the factory x4_400 fpga image flavor, through the python UHD package v4.6.0.0 (50fa3ba) on Ubuntu 22.04.