arm-bsp: remove armv8m

Musca B1 and S1 should be using the external tune files.  Remove armv8,
as it is not correct anyway, and use the Cortex-M33 tune.

Note:  This change will prevent glibc from compiling (and possibly
others).  This isn't relevant for a Cortex-M, as it should be
running an RTOS not Linux.

Change-Id: Id88d2af41217e8b5bad01e98b337cf0523acf6e1
Signed-off-by: Jon Mason <jon.mason@arm.com>