Add bitfield access macros

To use these, set things up like this:

#define UART_PA_START 0x67000000        Physical address of UART
struct uart_ctlr *uart = (struct uart_ctlr *)UART_PA_START;

#define UART_FBCON_RANGE  5:3           Bit range for the FBCON field
enum {                                  An enum with allowed values
        UART_FBCON_OFF,
        UART_FBCON_ON,
        UART_FBCON_MULTI,
        UART_FBCON_SLAVE,
};

This defines a bit field of 3 bits starting at bit 5 and extending down
to bit 3, i.e. 5:3

Then:
   bitfield_unpack(UART_FBCON)
        - return the value of bits 5:3 (shifted down to bits 2:0)

   bitfield_pack(UART_FBCON, 4)
        - return a word with that field set to 4 (so in this case (4 << 3))

   bitfield_set(UART_FBCON, word, val
        - update a field within word so that its value is val.

   bitfield_enum_writel(UART_FBCON, MULTI, &uart->fbcon)
        - set the UART's FBCON field to MULTI

Change-Id: Id90aa0538093d67c917df2d744c0ebff1fe03b6f

BUG=chromium-os:13875
TEST=cd test; make

Review URL: http://codereview.chromium.org/6883058

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