simple-bus: add DM_FLAG_PRE_RELOC flag to simple-bus driver

simple-bus: add DM_FLAG_PRE_RELOC flag to simple-bus driver

Boards such as qemu-riscv, which receive their device tree at runtime,
for example from QEMU or firmware, are unable to add the appropriate
device tree properties to make devices available pre relocation.
Instead, they must rely on the DM_FLAG_PRE_RELOC flag to be set for the
required drivers.

Add the DM_FLAG_PRE_RELOC flag to the simple-bus driver to make devices
under it with drivers that have set the flag as well available pre
relocation for these boards.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>