KVM: Documentation: Enumerate allowed value macros of `irq_type`

KVM: Documentation: Enumerate allowed value macros of `irq_type`

The expression `irq_type[n]` may confuse readers to interpret `n`
as the bit position and think of CPU = 1 << 0, SPI = 1 << 1, and
PPI = 1 << 2.

Since arch/arm64/include/uapi/asm/kvm.h already has macro definitions
for the allowed values, this commit uses these symbols to clear up
the ambiguity.

Signed-off-by: Changyuan Lyu <changyuanl@google.com>
Link: https://lore.kernel.org/r/20240623164542.2999626-2-changyuanl@google.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>