ARM: OMAP3: Fix booting with thumb2 kernel

We get a NULL pointer dereference on omap3 for thumb2 compiled kernels:

Internal error: Oops: 80000005 [#1] SMP THUMB2
...
[<c046497b>] (_raw_spin_unlock_irqrestore) from [<c0024375>]
(omap3_enter_idle_bm+0xc5/0x178)
[<c0024375>] (omap3_enter_idle_bm) from [<c0374e63>]
(cpuidle_enter_state+0x77/0x27c)
[<c0374e63>] (cpuidle_enter_state) from [<c00627f1>]
(cpu_startup_entry+0x155/0x23c)
[<c00627f1>] (cpu_startup_entry) from [<c06b9a47>]
(start_kernel+0x32f/0x338)
[<c06b9a47>] (start_kernel) from [<8000807f>] (0x8000807f)

The power management related assembly on omaps needs to interact with
ARM mode bootrom code, so we need to keep most of the related assembly
in ARM mode.

Turns out this error is because of missing ENDPROC for assembly code
as suggested by Stephen Boyd <sboyd@codeaurora.org>. Let's fix the
problem by adding ENDPROC in two places to sleep34xx.S.

Let's also remove the now duplicate custom code for mode switching.
This has been unnecessary since commit 6ebbf2ce ("ARM: convert
all "mov.* pc, reg" to "bx reg" for ARMv6+").

And let's also remove the comments about local variables, they are
now just confusing after the ENDPROC.

The reason why ENDPROC makes a difference is it sets .type and then
the compiler knows what to do with the thumb bit as explained at:

https://wiki.ubuntu.com/ARM/Thumb2PortingHowto



Reported-by: Kevin Hilman <khilman@kernel.org>
Tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>