ARM: shmobile: Remove shmobile_boot_arg

CPU boot configuration writes to shmobile_boot_arg, which is located in
the .text section, and thus should not be written to.

As of commit 1d33a354 ("ARM: shmobile: Per-CPU SMP boot / sleep
code for SCU SoCs"), and ignoring accidental remainings,
shmobile_boot_arg is always set to MPIDR_HWID_BITMASK by C code.
Hence we can just hardcode this in the assembler code, and remove the
variable, and thus also remove the need to write to this variable.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>