am43xx_evm: qspi: do not define DM_SPI and DM_SPI_FLASH for spl

Since OMAP's spl doesn't support DM currently, do not define
DM_SPI and DM_SPI_FLASH for spl build.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>