ARM: EXYNOS: Add support for secure monitor calls

Some boards use secure monitor calls to communicate with secure
firmware.

This patch adds exynos_smc function which uses smc assembly instruction
to do secure monitor calls.

Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>