Modified Sector Erase for AT91SAM4S

In FLASHD_ErasePages AT91C_EFC_FCMD_EPA is used to erase sectors.
According to the datasheet FARG[15:2] defines the page from which
the erase will start.This page must be modulo 4, 8, 16 or 32
according to the number of pages to erase. FARG[1:0] defines the
number of pages to be erased. Previously (firstpage << 2) was used
to conform to this, seems it should not be shifted... Changed it
to (firstPage) | erasePages.

Change-Id: I791cc7fc4faf056623ad5a6c7e860315306098a1
Signed-off-by: Olivier Schonken <olivier.schonken@gmail.com>
Reviewed-on: http://openocd.zylin.com/897


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>