configs: enable OSPI related configs in AM62x

Add am62x_evm_r5_defconfig for OSPI Flash support in R5 SPL
and am62x_evm_a53_defconfig for A53 SPL and U-Boot
support.
These configs enable OSPI Flash boot functionality in the board as well
as the usage of OSPI Flash from U-Boot.

Signed-off-by: Dhruva Gole <d-gole@ti.com>