m68k: Add missing ENTRY entry in linker script

m68k: Add missing ENTRY entry in linker script

Currently, there is no ENTRY() entry in the m68k linker script.
However, since they currently contain a version_string symbol, which
then overrides the default weak version_string symbol, the linker
decides we must keep start.o in the resulting link.  Add an ENTRY() line
so that the linker will know to keep this, even when version_string is
no longer provided in start.S

Cc: Huan Wang <alison.wang@nxp.com>
Cc: Angelo Dureghello <angelo@sysam.it>
Signed-off-by: Tom Rini <trini@konsulko.com>