mips: mt76xx: lowlevel_init.S: Add missing memory controller reset in DDR init

This fixes an issue which has been noticed on the Gardena board, with
the watchdog enabled, where the watdchdog reset (after a system hang)
did result in reporting of 2.9 GiB and a hang after this. With this
patch applied the memory controller is correctly reset and initialized
again even after a watchdog reset.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>