For the mainline u-boot you have to recompile it with
CONFIG_MMC0_CD_PIN=""
added to the config file (or set via menuconfig)
For the legacy kernel you have to change "sdc_detmode" in the fex file for your board (kernel code and some FEX files should have comments about possible values for this option)
For the mainline kernel you have to remove "cd-gpios" property and add "non-removable" or "broken-cd" properties to the mmc0 node - check Documentation/devicetree/bindings/mmc/mmc.txt in the kernel source for the details.
