imx8: sys_proto.h: change guard logic around ROM API

imx8: sys_proto.h: change guard logic around ROM API

This exposes the struct rom_api, the g_rom_api variable declaration
and the associated #defines to slightly fewer boards: namely, those
IMX8M which are not IMX8MN or IMX8MP. But the latter two are the only
IMX8M* ones where the g_rom_api variable is defined (in imx8m/soc.c),
so that should be fine.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>