arm64/gicv3: correct the size of the distributor resource

Use the GICD_SIZE macro (0x10000), which is half the size of the current
fixed-sized mapping (128 * 1024 == 0x20000).

In ARM64 Hyper-V instances, it seems the Distributor's registers are
located immediately preceding a range of physical memory in the bus
address space. Thus, when ram0 is attaching and attempts to reserve
SYS_RES_MEMORY resources corresponding to its physmem ranges, it fails,
because the first 0x10000 bytes of this range are already owned by gic0.

PR: 270415
Reported by: whu
Tested by: whu
Differential Revision: https://reviews.freebsd.org/D39260