riscv: Remove CONFIG_PAGE_OFFSET

The current definition of CONFIG_PAGE_OFFSET is problematic for a couple
of reasons:
 1) The value is misleading for normal 64-bit kernels, where it is
    overridden at runtime if Sv48 or Sv39 is chosen. This is especially
    the case for XIP kernels, which always use Sv39.
 2) The option is not user-visible, but for NOMMU kernels it must be a
    valid RAM address, and for !RELOCATABLE it must additionally be the
    exact address where the kernel is loaded.

Fix both of these by removing the option.
 1) For MMU kernels, drop the indirection through Kconfig. Additionally,
    for XIP, drop the indirection through kernel_map.
 2) For NOMMU kernels, use the user-visible physical RAM base if
    provided. Otherwise, force the kernel to be relocatable.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Jesse Taube <mr.bossman075@gmail.com>
Link: https://lore.kernel.org/r/20241026171441.3047904-7-samuel.holland@sifive.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>