I fixed it! My qmake.conf was correct but I was missing a configuration option. Now my configuration looks lit this:

~/qt-everywhere-src-5.11.2//configure -opensource -confirm-license -release -device stm32mp1 -device-option CROSS_COMPILE=~/gcc-linaro-7.4.1-2019.02-x86_64_arm-linux-gnueabihf/bin/arm-linux-gnueabihf- -sysroot ~/STM32MPU_workspace/STM32MP15-Ecosystem-v1.0.0/Developer-Package/SDK/sysroots/cortexa7t2hf-neon-vfpv4-openstlinux_weston-linux-gnueabi -skip qtwebengine -prefix /usr/local/Qt-5.11.2-arm -nomake examples -nomake tests -opengl es2