target/arm: Add the XML dynamic generation

target/arm: Add the XML dynamic generation

Generate an XML description for the cp-regs.
Register these regs with the gdb_register_coprocessor().
Add arm_gdb_get_sysreg() to use it as a callback to read those regs.
Add a dummy arm_gdb_set_sysreg().

Signed-off-by: Abdallah Bouassida <[email protected]>
Tested-by: Alex Bennée <[email protected]>
Message-id: 1524153386[email protected]
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>