kernel: support non-identity RAM mapping

Some platforms may have multiple RAM regions which are
dis-continuous in the physical memory map. We really want
these to be in a continuous virtual region, and we need to
stop assuming that there is just one SRAM region that is
identity-mapped.

We no longer use CONFIG_SRAM_BASE_ADDRESS and CONFIG_SRAM_SIZE
as the bounds of kernel RAM, and no longer assume in the core
kernel that these are identity mapped at boot.

Two new Kconfigs, CONFIG_KERNEL_VM_BASE and
CONFIG_KERNEL_RAM_SIZE now indicate the bounds of this region
in virtual memory.

We are currently only memory-mapping physical device driver
MMIO regions so we do not need virtual-to-physical calculations
to re-map RAM yet. When the time comes an architecture interface
will be defined for this.

Platforms which just have one RAM region may continue to
identity-map it.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>