staging: vc04_services: Convert dsb() to dsb(sy)

The dsb() macro for 32 arm compiles to dsb(sy) in the binary file.
This macro is no longer supported on arm64, so instead use dsb(sy)
which is completely binary compatible.

Signed-off-by: Michael Zoran <mzoran@crowfest.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>