configs: at91: remove SPL_GENERATE_ATMEL_PMECC_HEADER from non-nand configs

SPL_GENERATE_ATMEL_PMECC_HEADER will generate a header for the SPL for NAND
information. The initial stage 1 bootloader will use this header in case the
NAND flash doesn't support commands to retrieve sector size, etc.
However this header is bad for different boot media, like MMC or SPI.
In case SD_BOOT or SPI_BOOT is used, remove the config for this.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>