configs: am43xx_evm_defconfig: Enable CONFIG_SPI_FLASH_BAR

AM437x SK and AM437x IDK EVMs have 64MB flash, therefore enable
CONFIG_SPI_FLASH_BAR to access flash regions above 16MB.

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