devicetree: bindings: Add vendor prefix for Micron Technology, Inc.

This patch is used to add vendor prefix for Micron Technology, Inc. in
the vendor-prefixes.txt file.

Micron Technology, Inc. is an American multinational corporation based
in Boise, Idaho, best known for producing many forms of semiconductor
devices. This includes DRAM, SDRAM, flash memory, eMMC and SSDs.

Signed-off-by: Bean Huo <bpqw@micron.com>
[robh: cleanup commit msg formatting and company name]
Signed-off-by: Rob Herring <robh@kernel.org>