hw/intc/aspeed: Reduce regs array size by adding a register sub-region

hw/intc/aspeed: Reduce regs array size by adding a register sub-region

Currently, the size of the "regs" array is 0x2000, which is too large. So far,
it only uses "GICINT128 to `GICINT134", and the offsets from 0 to 0x1000 are
unused. To save code size and avoid mapping large unused gaps, update to only
map the useful set of registers:

INTC register [0x1000 – 0x1804]

Update "reg_size" to 0x808. Introduce a new class attribute "reg_offset" to set
the start offset of a "INTC" sub-region. Set the "reg_offset" to 0x1000 for INTC
registers.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250307035945.3698802-6-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>