If you need to test a custom version of grub2 you can follow the steps bellow to build the necessary packages for RHEL8 (it should also work on CentOS and Fedora):
subscription-manager register --username USER --password PWD --auto-attach
subscription-manager repos --enable codeready-builder-for-rhel-8-ppc64le-rpms
dnf update
dnf install dnf-utils
dnf groupinstall 'Development Tools'
yumdownloader --source grub2
rpm -ivh ./grub2-2.02-90.el8.src.rpm
dnf builddep ~/rpmbuild/SPECS/grub2.spec
scp -rp ./YOUR_FIX_OR_FEATURE.patch ~/rpmbuild/SOURCES/
vi ~/rpmbuild/SOURCES/grub.patches and add a new item at the end of the file:
Patch0318: YOUR_FIX_OR_FEATURE.patch
cd ./rpmbuild/
rpmbuild -bb ./SPECS/grub2.spec
cd ./RPMS/
cd ./ppc64le/
rpm -ivh --force ./*