bcm27xx: add kmod-rp1-pio-uart package

bcm27xx: add kmod-rp1-pio-uart package

Add a KernelPackage for CONFIG_SERIAL_RP1_PIO_UART, a software UART
implemented on the RP1 PIO block with DMA-driven TX/RX and a PIO
interrupt for break detection. Only 8N1 with no hardware flow control
is supported.

The package follows the same pattern as the other RP1 PIO consumers
(kmod-pwm-pio-rp1, kmod-ws2812-pio-rp1): it depends on kmod-rp1-pio
and is only available on bcm2712. Guard on 6.18, since the driver is
not available on 6.12.

Signed-off-by: Joshua Covington <joshuacov@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/24645
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>