firmware: arm_scpi: remove usage of drvdata and don't reset scpi_info to null

There's no benefit using drvdata as variable scpi_info is global.

Setting scpi_info to NULL in scpi_remove isn't needed too. If arm_scpi
is built-in, then this code is never used. And if arm_scpi is built as
a module and some other module calls get_scpi_ops() then due to this
dependency scpi_remove is called only after the other module has been
removed.

Last but not least, users usually store the result of get_scpi_ops(),
therefore setting scpi_info to NULL wouldn't really help.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
[sudeep.holla: reworded the commit log a bit]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>