board: ti: Introduce the basic files to support AM62 SK board

Add basic support for AM62 SK. This has 2GB DDR.
Note that stack for R5 SPL is in OCRAM @ 0x7000ffff so that is away from
BSS and does not step on BSS section

Add only the bare minimum required to support UART and SD.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>