riscv: Remove support for XIP kernel

riscv: Remove support for XIP kernel

XIP has a history of being broken for long periods of time. In 2023, it was
broken for 18 months before getting fixed [1]. In 2024 it was 4 months [2].

And now it is broken again since commit a44fb5722199 ("riscv: Add runtime
constant support"), 10 months ago.

These are clear signs that XIP feature is not being used.

I occasionally looked after XIP, but mostly because I was bored and had
nothing better to do.

Remove XIP support. Revert is possible if someone shows up complaining.

Link: https://lore.kernel.org/linux-riscv/20231212-customary-hardcover-e19462bf8e75@wendy/
Link: https://lore.kernel.org/linux-riscv/20240526110104.470429-1-namcao@linutronix.de/
Signed-off-by: Nam Cao <namcao@linutronix.de>
Cc: Frederik Haxel <haxel@fzi.de>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20260202115403.2119218-1-namcao@linutronix.de
[pjw@kernel.org: updated to apply]
Signed-off-by: Paul Walmsley <pjw@kernel.org>