ARM: shmobile: Let romImage rely on default ATAGS

Remove the ATAGS data structure from head-shmobile.S
since a default ATAGS is already provided by the code
in arch/arm/kernel/atags_parser.c. Passing a NULL as
ATAGS is valid. For actual hardware specific setup
the fixup callback in the board code may be used.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>