soc: bcm: brcmstb: Fix re-entry point with a THUMB2_KERNEL

When the kernel is built with CONFIG_THUMB2_KERNEL we would set the
kernel's resume entry point to be a function that is already built as
Thumb-2 code while the boot agent doing the resume is in ARM mode, so
this does not work. There is a header label defined: cpu_resume_arm
which we can use to do the switching for us.

Fixes: 0b741b82 ("soc: bcm: brcmstb: Add support for S2/S3/S5 suspend states (ARM)")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>