mt6765-common: Append fstab suffix to cmdline

[Sahil Sonar]
- R kernel as of now loads mt6762 in some instances instead of mt6765, to avoid copying duplicated fstab for mt6765 and mt6762 let's set postfix.
- Additionally, there are no references to mt6762 in cmdline.

Change-Id: I34fe5e40cd7eac3f5b482c97b54205e597122dc2