ARM: shmobile: Force ARM mode to compile reset vector for secondary CPUs

Instructions start from boot vector must be ARM mode.
This patch specify ARM mode explicitly and use 'bx' instruction to be
able to change to Thumb mode.

Signed-off-by: Tetsuyuki Kobayashi <koba@kmckk.co.jp>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>