staging: axis-fifo: add debugfs interface for dumping fifo registers

staging: axis-fifo: add debugfs interface for dumping fifo registers

For debugging purposes, add a simple, read-only debugfs interface to dump
the following fifo registers:
ISR  - Interrupt Status Register
IER  - Interrupt Enable Register
TDFV - Transmit Data FIFO Vacancy
RDFO - Receive Data FIFO Occupancy

$ cat /sys/kernel/debug/43c00000.axi_fifo_mm_s/regs
 isr: 0x00000000
 ier: 0xfe000000
tdfv: 0x000001fc
rdfo: 0x00000000

Signed-off-by: Ovidiu Panait <ovidiu.panait.oss@gmail.com>
Link: https://lore.kernel.org/r/20250720183833.3570345-2-ovidiu.panait.oss@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>