Dinu,
I hope you’ve not connect this PIN (EMA_A [15] / MMCSD0_DAT [6] / PRU1_R30 [23] / GP5 [15] / PRU1_R31 [23]) to any other device (Example: NOR) apart from controlling the Power module.
It should be in the Tristate mode (no external Pull-Up) unless you go and configure the GPIO Bank 5 register to enable this as output.
Ok you’re suspecting the UBL is doing some configuration with this pin .Have you tried other GPIO banks (Bank 8) by isolating the power module from the GP [15] just be curious.
Have you checked the AISgen utility for the proper boot mode settings (NAND Flash In your case) and configure the PINMUX11 Register for GP5 [15] and disable the SDRAM configuration option.
Regards
Antony