sifive: fu540: Add runtime boot mode detection

Add support to detect boot mode at runtime for
SiFive FU540 boards.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>