ARM: Fix section build warnings for AMBA drivers

Found in the Versatile build:

WARNING: drivers/built-in.o(.data+0x14c): Section mismatch in reference from the variable pl061_gpio_driver to the (unknown reference) .init.data:(unknown)
The variable pl061_gpio_driver references
the (unknown reference) __initdata (unknown)

WARNING: drivers/built-in.o(.data+0x40f8): Section mismatch in reference from the variable pl011_driver to the (unknown reference) .init.data:(unknown)
The variable pl011_driver references
the (unknown reference) __initdata (unknown)

WARNING: drivers/built-in.o(.data+0x5ab4): Section mismatch in reference from the variable pl031_driver to the (unknown reference) .init.data:(unknown)
The variable pl031_driver references
the (unknown reference) __initdata (unknown)

Basically, amba_id structures must not be __initdata.  Also fix:

WARNING: drivers/built-in.o(.data+0x138): Section mismatch in reference from the variable pl061_gpio_driver to the function .init.text:pl061_probe()
The variable pl061_gpio_driver references
the function __init pl061_probe()

which is an incorrectly annotated probe function.  Fix it to reflect
the other AMBA bus probe functions by removing the __init attributation.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>