rockchip: add /chosen/bootsource to U-Boot proper DT

rockchip: add /chosen/bootsource to U-Boot proper DT

U-Boot typically can be loaded from different storage media, such as
eMMC, SD card, SPI flash, but also from non-persistent media such as USB
(via proprietary protocols loading directly into SRAM, or fastboot, DFU,
 etc..), JTAG, ...

This information is usually reported by the BootROM via some proprietary
mechanism (some specific address in registers/DRAM for example). For
Rockchip, that information is stored in a register
(BROM_BOOTSOURCE_ID_ADDR).

While we already have the information about which medium was used to
load U-Boot proper from SPL (via /chosen/u-boot,spl-boot-device), this
new property represents the medium used to load U-Boot first phase
(depending on configuration, can be VPL/TPL/SPL) which absolutely may
differ from the one used to load U-Boot proper!

It would be useful to know which medium was used to load the first phase
of U-Boot, for example to check fallback mechanisms (proper loaded from
a different medium than first phase) are actually working.

For now, this only applies to Rockchip's U-Boot proper DT but could be
applied to the kernel's as well and possibly for other architectures or
vendors.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>