arm64/sysreg: Rename POE_RXW to POE_RWX

arm64/sysreg: Rename POE_RXW to POE_RWX

It is customary to list R, W, X permissions in that order. In fact
this is already the case for PIE constants (PIE_RWX). Rename POE_RXW
accordingly, as well as POE_XW (currently unused).

While at it also swap the W/X lines in
compute_s1_overlay_permissions() to follow the R, W, X order.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Link: https://lore.kernel.org/r/20250219164029.2309119-3-kevin.brodsky@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>