configs: Move CONFIG_FIT_SIGNATURE to defconfig

configs: Move CONFIG_FIT_SIGNATURE to defconfig

For the platforms which use,CONFIG_FIT_SIGNATURE, the required configs are
moved to the platform's defconfig file. Selecting CONFIG_FIT_SIGNATURE using
defconfig automatically resolves the dependencies for signature verification.
The RSA library gets automatically selected and user does not have to define
CONFIG_RSA manually.

Signed-off-by: Ruchika Gupta <[email protected]>
CC: Simon Glass <[email protected]>
Acked-by: Simon Glass <[email protected]>