drivers: serial: Don't condition uart_irq_rx_ready on irq enabled

The function uart_irq_rx_ready() should return true if there is data in
the receive buffer, regardless of whether the irq is enabled. Fix the
mcux and rv32m1 shim drivers to implement this behavior correctly.

Prior to this change:
- irq_rx_full() checks if RX data is available
- irq_rx_ready() checks if RX data is available and interrupts are
  enabled

After this change:
- irq_rx_full() checks if RX data is available
- irq_rx_ready() renamed to irq_rx_pending() to avoid confusion with the
  API ready() function
- API ready() implementation switched to use irq_rx_full()

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>