configs: Add spiboot support for am335x

am335x internal SRAM is too small to support the addition of
SPI bootmode to the default defconfig. Add a separate spiboot_defconfig

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
diff --git a/board/ti/am335x/MAINTAINERS b/board/ti/am335x/MAINTAINERS
index e100adf..219c871 100644
--- a/board/ti/am335x/MAINTAINERS
+++ b/board/ti/am335x/MAINTAINERS
@@ -5,3 +5,4 @@
 F:	include/configs/am335x_evm.h
 F:	configs/am335x_boneblack_vboot_defconfig
 F:	configs/am335x_evm_defconfig
+F:	configs/am335x_evm_spiboot_defconfig