configs: am62x_evm_a53_defconfig: Fix SF_DEFAULT_MODE

Setting sf default mode to 0x3 breaks sf update when we do SF read
through u-boot console.

This issue arises when we do a splash image through OSPI flash media,
to fix this set the default mode to 0x0.

Fixes: 04150400 ("configs: enable OSPI related configs in AM62x")
Signed-off-by: Nikhil M Jain <n-jain1@ti.com>