As others have said, the warnings can be ignored, but if they really bother you, then there are two ways of doing away with them.
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The first (and recommended) way is to install the package
mkinitcpio-firmwarefrom the AUR. This package will pull in all of the firmware modules reported in the warning that you’re seeing. -
The second way is to disable the creation of a fallback
initramfsin the preset(s) for your kernel(s). These presets can be found under/etc/mkinitcpio.d.
The reason why the first option is recommended is that many people have problematic hardware configurations, requiring the fallback initramfs for booting, and it is the generation of this fallback image which searches for the firmware modules that are missing on your system.
Personally I find it a bit weird that the mkinitcpio configuration from Arch upstream throws a warning about potentially missing firmware, while at the same time, the package that suppresses these warnings is not being made available via the official Arch repositories, and must be installed from the AUR instead. And this not a new development either, because those warnings have already been around for several years.
But, it is what it is, and above you can read how to remedy the problem. 