clocksource: samsung-time: Remove use of static register mapping

This patch brings the samsung-time driver one step closer to
multiplatform by replacing references to statically mapped registers
with dynamic mapping using ioremap.

The helper struct samsung_timer_variant is extended with reg_base field,
which is used to pass platform-specific base address to the driver.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
clocksource: samsung-time: Use clk_get_sys for getting clocks

This patch removes the need to reference platform_device structs defined
in platform code, by using clk_get_sys instead of clk_get for getting
necessary clocks.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>