rk3399: m0: Fix compiler warnings.

GCC complains for quite some versions, when compiling the M0 firmware
for Rockchip's rk3399 platform, about an invalid type of function 'main':

  warning: return type of 'main' is not 'int' [-Wmain]

This patch addresses this, by renaming the function to 'm0_main'.

Signed-off-by: Christoph Müllner <christophm30@gmail.com>
Change-Id: I10887f2bda6bdb48c5017044c264139004f7c785