clk: Add Fixed MMIO clock driver

This patch adds a driver for Fixed MMIO clock.
The driver reads a clock frequency value from a single 32-bit memory
mapped register and registers it as a fixed rate clock.

It can be enabled with COMMON_CLK_FIXED_MMIO Kconfig option.

Signed-off-by: Jan Kotas <jank@cadence.com>
[sboyd@kernel.org: Make of_fixed_mmio_clk_setup() static, use clk_hw
based APIs]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>