The main point is to save kernel RAM (which is not paged). Rather than building a kernel with everything in it, the kernel is built with some important drivers in it, and the rest available as modules. At boot time the kernel probes the various buses it finds, like pcie, and tries to find modules which claim to be drivers for the various things found. These ...
Make sure that the kernel modules that drive the TPM get loaded within initramfs by listing them in /etc/initramfs-tools/modules. Then the initramfs udev should create the devices for you.
First, run lsmod | grep tpm to find your TPM driver module(s). For me, the output looks like this:
# lsmod |grep tpm
tpm_tis 16384 0
Regenerate a new initramfs-linux.img accordingly to the archlinux wiki:
mkinitcpio -c /etc/mkinitcpio.conf -g /boot/initramfs-linux.img -k 5.5.7-arch1-1
use ls /usr/lib/modules to get the available kernel.