xtensa: implement ndelay

Proper ndelay implementation allows for faster IO rate with drivers that
use ndelay to access their device registers, as otherwise ndelay is
emulated with udelay.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

xtensa: clean up udelay

Replace division with shift, use ccount_freq instead of loops_per_jiffy.
Introduce __MAX_UDELAY and (undefined) __bad_udelay, break build for too
big udelay constants.
Remove irrelevant comment, clean up code style.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

xtensa: fix __delay for small loop count

Avoid __delay counter underflow for loop counts < 2.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>

xtensa: consolidate ccount access routines

Use get_ccount everywhere; remove xtensa_get_ccount.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Chris Zankel <chris@zankel.net>

xtensa: clean up files to make them code-style compliant

Remove heading and trailing spaces, trim trailing lines, and wrap lines
that are longer than 80 characters.

Signed-off-by: Chris Zankel <chris@zankel.net>

xtensa: reorganize SR referencing

- reference SRs by names where possible, not by numbers;
- get rid of __stringify around SR names where possible;
- remove unneeded SR names from asm/regs.h;
- add SREG_ prefix to remaining SR names;

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>

xtensa: move headers files to arch/xtensa/include

Move all header files for xtensa to arch/xtensa/include and platform and
variant header files to the appropriate arch/xtensa/platforms/ and
arch/xtensa/variants/ directories.

Moving the files gets also rid of all uses of symlinks in the Makefile.

This has been completed already for the majority of the architectures
and xtensa is one out of six missing.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Chris Zankel <chris@zankel.net>