modules: silabs: Link against RAIL Multiprotocol blobs when enabled

Enables RAIL Multiprotocol support by linking against the multiprotocol
RAIL library blob when `CONFIG_SILABS_GECKO_RAIL_MULTIPROTOCOL` or
`CONFIG_SILABS_SISDK_RAIL_MULTIPROTOCOL` are enabled.

Signed-off-by: James Smith <james@loopj.com>