exynos: allow SPL to build in thumb mode

Building peach-pi smdk5420 and peach-pit with thumb mode for SPL
ends-up in the following error:

Error: Thumb encoding does not support an immediate here -- `msr cpsr_c,#0x13|0xC0'

Use an intermediate register to be able to use thumb for exynos5 SPL.

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>

Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Tom Rini <trini@konsulko.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>