The xhci_pci problem seems to have recurred with kernel 5.14:
==> Building image from preset: /etc/mkinitcpio.d/linux514.preset: 'default'
-> -k /boot/vmlinuz-5.14-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.14-x86_64.img
==> Starting build: 5.14.0-1-MANJARO
-> Running build hook: [base]
-> Running build hook: [systemd]
-> Running build hook: [autodetect]
-> Running build hook: [modconf]
-> Running build hook: [block]
==> WARNING: Possibly missing firmware for module: xhci_pci
-> Running build hook: [keyboard]
-> Running build hook: [keymap]
-> Running build hook: [filesystems]
==> Generating module dependencies
==> Creating gzip-compressed initcpio image: /boot/initramfs-5.14-x86_64.img
==> Image generation successful
==> Building image from preset: /etc/mkinitcpio.d/linux514.preset: 'fallback'
-> -k /boot/vmlinuz-5.14-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.14-x86_64-fallback.img -S autodetect
==> Starting build: 5.14.0-1-MANJARO
-> Running build hook: [base]
-> Running build hook: [systemd]
-> Running build hook: [modconf]
-> Running build hook: [block]
==> WARNING: Possibly missing firmware for module: xhci_pci
-> Running build hook: [keyboard]
-> Running build hook: [keymap]
-> Running build hook: [filesystems]
==> Generating module dependencies
==> Creating gzip-compressed initcpio image: /boot/initramfs-5.14-x86_64-fallback.img
==> Image generation successful
See also Xhci_pci is missed in Linux Kernel 5.8 for previous kernels. I assume that this is a similar problem.
UPDATE:
From one of the links in that thread, it looks like installing upd72020x-fw from AUR suppresses this error message.