Due to a problem in the Arm Trusted Firmware (ATF) version 2.4.0 and earlier, when uart1 is configured for U-boot, the HPS will hang.
To work around this problem in the ATF version 2.4.0 and eariler, you could make a modification to use the real register address for UART1 in arm-trusted-frimware/plat/intel/soc/common/include/platform_def.h.
Here is an example for Intel Agilex® SoC devices (macro PLAT_UART0_BASE used across the ATF code, a simple modification is suggested here):
Change
#define PLAT_UART0_BASE (0xFFC02000)
to
#define PLAT_UART0_BASE (0xFFC02100)
This problem is fixed starting with the ATF version 2.4.1.