boards: Add config to mimxrt10{20,60,64}_evk boards to link into sdram

Adds a new configuration option to mimxrt10{20,60,64}_evk boards to link
data into external sdram. The default remains to link data into internal
data tightly coupled memory (DTCM).

Note that mimxrt1050_evk is not included because it already has support
for linking data into sdram.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>