tty: serial: amba: Use linux/{bits,bitfield}.h macros

The driver uses bit shifts and hexadecimal expressions to declare
constants. Replace that with the BIT(), GENMASK() & FIELD_PREP_CONST()
macros to clarify intent.

include/linux/amba/serial.h gets included from arch/arm/include/debug/pl01x.S.
Avoid includes and macro tricks for the four defines that are involved:
UART01x_DR, UART01x_FR, UART01x_FR_TXFF and UART01x_FR_BUSY.
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Link: https://lore.kernel.org/r/20231207-mbly-uart-v6-1-e384afa5e78c@bootlin.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>