arm64/sysreg: Move POR_EL0_INIT to asm/por.h

arm64/sysreg: Move POR_EL0_INIT to asm/por.h

The value of POR_EL0_INIT is not architectural, it is a software
decision. Since we have a dedicated header for POR_ELx, we might as
well define POR_EL0_INIT there.

While at it also define POR_EL0_INIT using POR_ELx_PERM_PREP(),
making it clearer that we are setting permissions for POIndex/pkey
0.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Link: https://lore.kernel.org/r/20250219164029.2309119-4-kevin.brodsky@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>