Add README for CPUID kernel modules for ARM

drivers/arm/freebsd/README.md

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+# CPUID FreeBSD kernel module for ARM
+
+This kernel module allows to:
+- access AArch32 registers in AArch32 state by using the [`MRC` instruction](https://developer.arm.com/documentation/dui0489/i/arm-and-thumb-instructions/mrc-and-mrc2)
+- access both AArch32 and AArch64 registers in AArch64 state by using the [`MRS` instruction](https://developer.arm.com/documentation/dui0473/m/arm-and-thumb-instructions/mrs--system-coprocessor-register-to-arm-register-)
+
+## Build
+
+The module is built during `cmake --build build` and present in the build directory. You can load it by using:
+```shell
+kldload build/drivers/arm/freebsd/cpuid.ko
+```

drivers/arm/linux/README.md

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+# CPUID Linux kernel module for ARM
+
+This kernel module allows to:
+- access AArch32 registers in AArch32 state by using the [`MRC` instruction](https://developer.arm.com/documentation/dui0489/i/arm-and-thumb-instructions/mrc-and-mrc2)
+- access both AArch32 and AArch64 registers in AArch64 state by using the [`MRS` instruction](https://developer.arm.com/documentation/dui0473/m/arm-and-thumb-instructions/mrs--system-coprocessor-register-to-arm-register-)
+
+## Build
+
+### Without DKMS
+
+Building the kernel module by using CMake is useful for libcpuid developers. This method is discouraged for end-users, because you will need to rebuild this module on every kernel updates.
+
+To build the kernel module in the build directory, use CMake with the `-DLIBCPUID_DRIVER_ARM_LINUX_DKMS=OFF` option.  
+Then, you can load the module by using:
+```shell
+# insmod build/drivers/arm/linux/cpuid.ko
+```
+
+### With DKMS
+
+Once the project is installed (`cmake --install build`), DKMS files will be present under `/usr/src`.  
+Use the `dkms` command to build the module, and replace `X.Y.Z` with libcpuid version:
+```shell
+# dkms install -m cpuid -v X.Y.Z
+# modprobe cpuid
+```