configs: Add configs for AM62x SK

Add am62x_evm_r5_defconfig for R5 SPL and am62x_evm_a53_defconfig for
A53 SPL and U-Boot support.

To keep the changes to minimum. Only UART And SD boot related configs
are included. This should serve as good starting point for new board
bringup with AM62x.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
[trini: Migrate a number of CONFIG symbols, have re-tested]
Tested-by: Georgi Vlaev <g-vlaev@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>