arm64: Fix off-by-one in its_init_cpu_lpi

The low bits of GICR_PROPBASER are defined as

The number of bits of LPI INTID supported, minus one, by the LPI
Configuration table starting at Physical_Address.

but flsl(1 << n) returns n + 1; use ilog2_long instead.

PR: 285677
Reported by: Julien Grall
Reviewed by: andrew
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D49542