drivers/serial/uart_pl011.c : add the interface about clock and reset control.

[TakuyaMiyasita]

Summary

I want to add the interface about clock and reset control for reducing power consumption.
I added the pl011_platform_shutdown() / _setup(), these functions are called when pl011_shutdown() / _setup() are called.

The added interface is inteneded as bellows.

• The pl011_platform_shutdown() is for clock off and reset-asserted.
• The pl011_platform_setup() is for clock on and reset-de-asserted.

and

This PR includes as following fixes.

• bug fix about the function : pl011_txready().
• add the bitmask according to PL011 spec. (Although almost no problem)

Impact

The user of drivers/serial/uart_pl011.c
I think almost no impact as long as do not use CONFIG_UART_PL011_PLATFORMIF.

Testing

I checked following items with our out-of-tree Board directory, some apps and cxd32xx. (The PR of cxd32xx have not accepted yet.)

• Boot NuttShell(NSH)
• Passs the "apps/testing/ostest"

[jerpelea]

please add commit message describing the change