firmware: Bring back FW_TEXT_START as an optional parameter

Bring back FW_TEXT_START as an optional parameter to allow users
explicitly specify compile time address for loading debug symbols.
When not specified, the FW_TEXT_START is assumed to be 0.

Fixes: d4d2582e

 ("firmware: remove FW_TEXT_START")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Tested-by: Clément Léger <cleger@rivosinc.com>