Please support BCM2711

[TyphoonWu]

I found Pi4 MEM_FLAG = 0x04
#elif (RASPI) == 4 // Raspberry Pi 4
#define PERIPH_VIRT_BASE 0xfe000000
#define PERIPH_PHYS_BASE 0x7e000000
#define DRAM_PHYS_BASE 0xc0000000
#define MEM_FLAG 0x04
#define XTAL_CLOCK 54.0e6

So in dma.cpp please add BCM2711
if (strstr(modelstr, "BCM2708"))
hw.board_model = 1;
else if (strstr(modelstr, "BCM2709") || strstr(modelstr, "BCM2835"))
hw.board_model = 2;
else
fatal("rpio-pwm: Cannot parse the hardware name string\n");