ARM: 8031/1: fixmap: remove FIX_KMAP_BEGIN and FIX_KMAP_END

It seems that these two macros are not used by non architecture
specific code. And on ARM FIX_KMAP_BEGIN equals zero.

This patch removes these two macros. Instead, using FIX_KMAP_NR_PTES to
tell the pte number belonged to fixmap mapping region. The code will
become clearer when I introduce a bugfix on fixmap mapping region.

Reviewed-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Liu Hua <sdu.liu@huawei.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>