riscv: Get rid of CONFIG_PHYS_RAM_BASE in kernel physical address conversion

The usage of CONFIG_PHYS_RAM_BASE for all kernel types was a mistake:
this value is implementation-specific and this breaks the genericity of
the RISC-V kernel.

Fix this by introducing a new variable phys_ram_base that holds this
value at runtime and use it in the kernel physical address conversion
macro. Since this value is used only for XIP kernels, evaluate it only if
CONFIG_XIP_KERNEL is set which in addition optimizes this macro for
standard kernels at compile-time.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Tested-by: Emil Renner Berthing <kernel@esmil.dk>
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Fixes: 44c92257 ("RISC-V: enable XIP")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>