samples: subsys: fs: Refactor fat_fs sample yaml to reduce whitelist

Refactors the fat_fs sample yaml to depend on a feature rather than
whitelisting specific boards. This implicitly extends the sample to
mimxrt10{60,64}_evk boards, since they already support the feature.

The only whitelist board remaining is the nrf52840_blip, which requires
a device tree overlay for this sample.

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