samples: openamp_rsc_table: move arch specific config from project

The CONFIG_PLATFORM_SPECIFIC_INIT is ARM specific, so move this to
board config file, rather than project config.
It should be no restriction to run the openamp_rsc_table sample on other,
non-arm, platforms.

Signed-off-by: Iuliana Prodan <iuliana.prodan@nxp.com>