x86: Avoid magic number with ELCR register accesses

Define PIC_ELCR1 and PIC_ELCR2 macros for accesses to the ELCR registers 
implemented by many chipsets in their embedded 8259A PIC cores, avoiding 
magic numbers that are difficult to handle, and complementing the macros 
we already have for registers originally defined with discrete 8259A PIC 
implementations.  No functional change.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2107200237300.9461@angie.orcam.me.uk