rockchip: spl: Add a read_brom_bootsource_id() helper

rockchip: spl: Add a read_brom_bootsource_id() helper

The bootsource ids reported by BootROM of RK3528 and RK3576 for e.g.
SPI NOR and USB differs slightly compared to prior SoCs:

- Booting from sfc0 (ROCK 4D) report the normal bootsource id 0x3.
- Booting from sfc1 M1 (NanoPi M5) report a new bootsource id 0x23.
- Booting from sfc1 M0 has not been tested (no board using this config).
- Booting from USB report a new bootsource id 0x81 on RK3528 and RK3576.

Add a helper function to read the bootsource id. This helper function
will be used to translate the new values to the common BROM_BOOTSOURCE
enum values on RK3528 and RK3576.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>