xen/arm: acpi: The boot CPU does not always match the first entry in the MADT

xen/arm: acpi: The boot CPU does not always match the first entry in the MADT

Since the ACPI 6.0 errata document [1], the first entry in the MADT
does not have to correspond to the boot CPU.

Introduce a new variable to know if a MADT entry matching the boot CPU
is found. Furthermore, it's not necessary to check if the MPIDR is
duplicated for the boot CPU. So the rest of the function can be skipped.

[1] 1380 Unnecessary restrictions to FW vendors in ordering of GIC structures
in MADT

Signed-off-by: Julien Grall <julien.grall@arm.com>