modules: mcux: introduce CONFIG_MCUX_CORE_SUFFIX

MCUX SDK depends on "core suffixes" in order to identify which core a
build is targeting on a multicore part. Previously, this information was
parsed from the CONFIG_SOC string, but with hardware model v2 this is no
longer possible. Introduce the Kconfig MCUX_CORE_SUFFIX, which multicore
SOCs can set to inform MCUX which core the build is targeting.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>