remoteproc/mediatek: Remove non-standard dsb()

As reported by Stephen, dsb() is not declared on e.g. x86_64, preventing
the mtp_scp from building. Simply remove the barrier (and the readback),
suggested by Pi-Hsun to resolve this.

Fixes: fd0b6c1f

 ("remoteproc/mediatek: Add support for mt8192 SCP")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Suggested-by: Pi-Hsun Shih <pihsun@chromium.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>