esirisc_flash: Rename PAGE_SIZE to FLASH_PAGE_SIZE

esirisc_flash: Rename PAGE_SIZE to FLASH_PAGE_SIZE

PAGE_SIZE is defined in system includes on some systems, this would
avoid the unintended conflict

Fixes
| src/flash/nor/esirisc_flash.c:95:9: error: 'PAGE_SIZE' macro redefined [-Werror,-Wmacro-redefined]
| #define PAGE_SIZE                       4096
|         ^
| /mnt/a/yoe/build/tmp/work/core2-64-yoe-linux-musl/openocd/0.10+gitrAUTOINC+7ee618692f-r0/recipe-sysroot/usr/inclu
de/limits.h:89:9: note: previous definition is here
| #define PAGE_SIZE PAGESIZE

Change-Id: I195b303fc88a7c848ca4e55fd6ba893796df55cc
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Reviewed-on: http://openocd.zylin.com/5180
Tested-by: jenkins
Reviewed-by: Steven Stallion <sstallion@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>