riscv: Fix build error on rv32 + XIP

riscv: Fix build error on rv32 + XIP

commit 66f1e6809397 ("riscv: Make XIP bootable again") restricted page
offset to the sv39 page offset instead of the default sv57, which makes
sense since probably the platforms that target XIP kernels do not
support anything else than sv39 and we do not try to find out the
largest address space supported on XIP kernels (ie set_satp_mode()).

But PAGE_OFFSET_L3 is not defined for rv32, so fix the build error by
restoring the previous behaviour which picks CONFIG_PAGE_OFFSET for rv32.

Fixes: 66f1e6809397 ("riscv: Make XIP bootable again")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Closes: https://lore.kernel.org/linux-riscv/344dca85-5c48-44e1-bc64-4fa7973edd12@infradead.org/T/#u
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Link: https://lore.kernel.org/r/20240118212120.2087803-1-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>