Does MMU starts functioning from the device boot or the kernel starting point.?
I happen to have the i.MX53 reference manual open, which says very little about the MMU except this paragraph:


  Boot ROM includes a feature of enabling the Memory Management Unit (MMU) and
  caches to improve boot speed when performing a secure boot with
  SEC_CONFIG=Closed ( High Assurance Boot (HAB)). L1 instruction cache is enabled
  at the start of image download. L1 data cache, L2 cache and MMU are enabled during
  image authentication.


So the MMU starts 'off' or in a transparent mode, where all addresses are treated as physical addresses. The boot ROM in this particular chip can establish a simple mapping and turn it on, which is required to use the cache: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka13835.html

(High Assurance Boot will run quite a bit more code than the regular bootloader in order to verify the signature of the operating system before handing over control to it.)

Once the operating system has been loaded, it will re-initialize the MMU to its own preferences early on in the boot sequence.