Documentation: devicetree: Update Exynos MCT bindings description

This patch updates description of device tree bindings for Exynos MCT
(multicore timers). Namely:
 - added note about simplified specification of local timer interrupts,
   when using single per-processor interrupt for all local timers,
 - changed first example that was incorrectly suggesting that global
   timer interrupts are optional,
 - simplified example interrupt map,
 - added example showing simplified local timer interrupt specification.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>