soc/mediatek: Add back SOF-only entry point

I thought I was being clever letting the linker place the entry point
arbitrarily (since the hardware can set it to any value).

But it turns out that the upstream Linux SOF loader code is hard-wired
to start the DSP only at the first byte of SRAM, always, no matter
what entry point is listed in the rimage file.  So until/unless this
is fixed, we need to add a trampoline at the start of SRAM (and
frustratingly that needs to be 1024 bytes long becuase of the
alignment requirements of the vector table that follows it, sigh...)

Signed-off-by: Andy Ross <andyross@google.com>